thickness (n.) Look up thickness at Dictionary.com
Old English þicness "density, viscosity, hardness; depth; anything thick or heavy; darkness; thicket;" see thick + -ness.
thickset (adj.) Look up thickset at Dictionary.com
also thick-set, late 14c., thikke sette "with parts or things set close together" (of grass on a sward, etc.), from thick + set (v.). Meaning "stocky, strong and square-built" is recorded from 1724.
thief (n.) Look up thief at Dictionary.com
Old English þeof "thief, robber," from Proto-Germanic *theubaz (cognates: Old Frisian thiaf, Old Saxon thiof, Middle Dutch and Dutch dief, Old High German diob, German dieb, Old Norse þiofr, Gothic þiufs), of uncertain origin.
thieve (v.) Look up thieve at Dictionary.com
Old English þeofian "to thieve, steal," from þeof (see thief). Rare in Old English, rarer in Middle English, not common until 17c.; perhaps the modern word is a late 16c. re-formation. Thieving (adj.) first attested 1520s.
thievery (n.) Look up thievery at Dictionary.com
1560s, from thieve + -ery. An Old English word for it was þeofend.
thievish (adj.) Look up thievish at Dictionary.com
mid-15c., "of or pertaining to thieves," from thieve or thief + -ish. Meaning "inclined to steal" is from 1530s. Wyclif and Chaucer used thiefly (late 14c.). Related: Thievishly; thievishness.
thigh (n.) Look up thigh at Dictionary.com
Old English þeoh, þeh, from Proto-Germanic *theuham (cognates: Old Frisian thiach, Old Dutch thio, Dutch dij, Old Norse þjo, Old High German dioh), probably literally "the thick or fat part of the leg," from PIE *teuk- from root *teue- (2) "to swell" (cognates: Lithuanian taukas, Old Church Slavonic tuku, Russian tuku "fat of animals;" Lithuanian tukti "to become fat;" Avestan tuma "fat;" Greek tylos "callus, lump," tymbos "burial mound, grave, tomb;" Old Irish ton "rump;" Latin tumere "to swell," tumulus "raised heap of earth," tumidus "swollen;"tumor "a swelling;" Middle Irish tomm "a small hill," Welsh tom "mound").
thigmotropism (n.) Look up thigmotropism at Dictionary.com
1900, from thigmo-, comb. form meaning "touch," from Greek thigma "touch" + tropism.
thilk (pron., adj.) Look up thilk at Dictionary.com
"that same, the very thing," early 13c., contraction from þe "the" (see the) + ilce "same" (see ilk).
thimble (n.) Look up thimble at Dictionary.com
Old English þymel "sheath or covering for the thumb," from thuma (see thumb (n.)) + instrumental suffix -el (1), used in forming names of tools (compare handle (n.)). The excrescent -b- appears mid-15c. (compare humble, nimble). Originally of leather, metal ones came into use 17c. Related: Thimbleful. Thimblerig, con game played with three thimbles and a pea or button, is attested from 1825 by this name, though references to thimble cheats, probably the same swindle, date back to 1716 (see rig (v.)).
thin (adj.) Look up thin at Dictionary.com
Old English þynne "narrow, lean, scanty, not dense; fluid, tenuous; weak, poor," from Proto-Germanic *thunni "thin" (cognates: West Frisian ten, Middle Low German dunne, Middle Dutch dunne, Dutch dun, Old High German dunni, German dünn, Old Norse þunnr, Swedish tunn, Danish tynd), from PIE *tnu- "stretched, stretched out" (hence "thin"), from root *ten- "to stretch" (cognates: Latin tenuis "thin, slender;" see tenet).
These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre. [Shakespeare, "The Tempest," IV.i.150, 1610]
"Loose or sparse," hence "easily seen through," with figurative extensions. Related: Thinly; thinness. Thin-skinned is attested from 1590s; the figurative sense of "touchy" is from 1670s.
thin (v.) Look up thin at Dictionary.com
Old English þynnian "to make thin, lessen, dilute," also intransitive, "become thin," from thin (adj.). Intransitive sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Compare similarly formed German dünnen, Dutch dunnen. Related: Thinned; thinning.
thine (pron.) Look up thine at Dictionary.com
Old English þin, possessive pronoun (originally genitive of þu "thou"), from Proto-Germanic *thinaz (cognates: Old Frisian, Old Saxon thin, Middle Dutch dijn, Old High German din, German dein, Old Norse þin), from PIE *t(w)eino-, suffixed form of second person singular pronominal base *tu-. A brief history of the second person pronoun in English can be found here; see also thou.
thing (n.) Look up thing at Dictionary.com
Old English þing "meeting, assembly, council, discussion," later "entity, being, matter" (subject of deliberation in an assembly), also "act, deed, event, material object, body, being, creature," from Proto-Germanic *thingam "assembly" (cognates: Old Frisian thing "assembly, council, suit, matter, thing," Middle Dutch dinc "court-day, suit, plea, concern, affair, thing," Dutch ding "thing," Old High German ding "public assembly for judgment and business, lawsuit," German Ding "affair, matter, thing," Old Norse þing "public assembly"). The Germanic word is perhaps literally "appointed time," from a PIE *tenk- (1), from root *ten- "stretch," perhaps on notion of "stretch of time for a meeting or assembly."

The sense "meeting, assembly" did not survive Old English. For sense evolution, compare French chose, Spanish cosa "thing," from Latin causa "judicial process, lawsuit, case;" Latin res "affair, thing," also "case at law, cause." Old sense is preserved in second element of hustings and in Icelandic Althing, the nation's general assembly.

Of persons, often pityingly, from late 13c. Used colloquially since c. 1600 to indicate things the speaker can't name at the moment, often with various meaningless suffixes (see thingamajig). Things "personal possessions" is from c. 1300. The thing "what's stylish or fashionable" is recorded from 1762. Phrase do your thing "follow your particular predilection," though associated with hippie-speak of 1960s is attested from 1841.
thingamajig (n.) Look up thingamajig at Dictionary.com
also thingumajig, 1824, see thing. Compare in similar sense kickumbob (1620s), thingum (1670s), thingumbob (1751), thingummy (1796), jigamaree (1824).
think (v.) Look up think at Dictionary.com
Old English þencan "imagine, conceive in the mind; consider, meditate, remember; intend, wish, desire" (past tense þohte, past participle geþoht), probably originally "cause to appear to oneself," from Proto-Germanic *thankjan (cognates: Old Frisian thinka, Old Saxon thenkian, Old High German denchen, German denken, Old Norse þekkja, Gothic þagkjan).

Old English þencan is the causative form of the distinct Old English verb þyncan "to seem, to appear" (past tense þuhte, past participle geþuht), from Proto-Germanic *thunkjan (cognates: German dünken, däuchte). Both are from PIE *tong- "to think, feel" which also is the root of thought and thank.

The two Old English words converged in Middle English and þyncan "to seem" was absorbed, except for its preservation in archaic methinks "it seems to me." As a noun, "act of prolonged thinking," from 1834. The figurative thinking cap is attested from 1839.
think tank (n.) Look up think tank at Dictionary.com
also think-tank, 1959 as "research institute" (first reference is to Center for Behavioral Sciences, Palo Alto, Calif.); it had been colloquial for "the brain" since 1905. See think + tank (n.).
thinkable (adj.) Look up thinkable at Dictionary.com
1805; see think (v.) + -able. Possibly a back-formation from unthinkable.
thinker (n.) Look up thinker at Dictionary.com
"one who has cultivated the powers of thought," mid-15c., agent noun from think (v.).
thinner (n.) Look up thinner at Dictionary.com
liquid used to dilute paint, ink, etc., 1904, agent noun from thin (v.).
third (adj.) Look up third at Dictionary.com
late Old English metathesis of þridda, from Proto-Germanic *thridja- (cognates: Old Frisian thredda, Old Saxon thriddio, Middle Low German drudde, Dutch derde, Old High German dritto, German dritte, Old Norse þriðe, Danish tredie, Swedish tredje, Gothic þridja), from PIE *tri-tyo- (cognates: Sanskrit trtiyas, Avestan thritya, Greek tritos, Latin tertius (source of Italian terzo, Spanish tercio, French tiers), Old Church Slavonic tretiji, Lithuanian trecias, Old Irish triss, Welsh tryde), suffixed form of root *trei- (see three).

Metathesis of thrid into third is attested from c.950 in Northumbrian, but overall thrid was prevalent up to 16c. The noun meaning "third part of anything" is recorded from late 14c. Third rail in electric railway sense is recorded from 1890. Third World War as a possibility first recorded 1947. Third-rate "of poor quality" is from 1814, ultimately from classification of ships (1640s); third class in railway travel is from 1839. Third Reich (1930) is a partial translation of German drittes Reich (1923). Third party in law, insurance, etc., is from 1818.
third degree (n.) Look up third degree at Dictionary.com
"intense interrogation by police," 1900, probably a reference to Third Degree of master mason in Freemasonry (1772), the conferring of which included an interrogation ceremony. Third degree as a measure of severity of burns (most severe) is attested from 1866, from French (1832); in American English, as a definition of the seriousness of a particular type of crime (the least serious type) it is recorded from 1865.
Third World (n.) Look up Third World at Dictionary.com
1963, from French tiers monde, formulated 1952 by French economic historian Alfred Sauvy (1898-1990) on model of the third estate (French tiers état) of Revolutionary France; his first world (The West) and second world (the Soviet bloc) never caught on.
thirst (v.) Look up thirst at Dictionary.com
Old English þyrstan "to thirst, thirst after," from the noun (see thirst (n.)); the figurative sense of the verb was present in Old English. Compare Old Saxon thurstian, Dutch dorsten, Old High German dursten, German dürsten, all verbs from nouns. Related: Thirsted; thirsting.
thirst (n.) Look up thirst at Dictionary.com
Old English þurst, from Proto-Germanic *thurstu- (cognates: Old Saxon thurst, Frisian torst, Dutch dorst, Old High German and German durst), from Proto-Germanic verbal stem *thurs- (cognates: Gothic thaursjan, Old English thyrre), from PIE root *ters- "dry" (see terrain). Figurative sense of "vehement desire" is attested from c. 1200.
thirsty (adj.) Look up thirsty at Dictionary.com
Old English þurstig "thirsty, greedy;" see thirst (n.) + -y (2). Related: Thirstily; thirstiness. Similar formation in Old Frisian, Dutch dorstig, German durstig.
thirteen (adj.) Look up thirteen at Dictionary.com
late 145c., metathesis of Middle English thrittene, from Old English þreotene (Mercian), þreotiene (West Saxon), from þreo "three" (see three) + -tene (see -teen). Similar formation in Old Saxon thriutein, Old Frisian thretten, Dutch dertien, German dreizehn, Old Norse threttan, Swedish tretton. As a noun from late Old English.

Not an unlucky number in medieval England, but associated rather with the customary "extra item" (as in baker's dozen). Superstitions began with association with the Last Supper, and the unluckiness of 13 sitting down together to dine (attested from 1690s). Most of the modern superstitions (buildings with floor "12-A," etc.) have developed since 1890.
thirteenth (adj.) Look up thirteenth at Dictionary.com
15c. metathesis of þriteenþe (mid-14c.; see thirteen + -th (1)), which replaced forms from Old English þreoteoða (West Saxon), þriteogeða (Anglian). Similar formation in Old Norse þrettande, Danish trettende, Swedish trettonde, Old Frisian threttinde, Dutch dertiende, Old High German dritto-zehanto, German dreizehnte.
thirties (n.) Look up thirties at Dictionary.com
1827 as the years of someone's life between 30 and 39; 1830 as the fourth decade of years in a given century. See thirty.
thirtieth Look up thirtieth at Dictionary.com
Middle English threttyth, from Old English þritigoða; see thirty + -th (1). Respelled 16c. to conform to new spelling of thirty.
thirty (adj.) Look up thirty at Dictionary.com
late 14c. metathesis of thritti, from Old English þritig, from þri, þreo "three" (see three) + -tig "group of ten" (see -ty (1)). Similar formation in Old Frisian thritich, Old Saxon thritig, Dutch dertig, Old High German drizzug, German dreissig.

The Thirty Years' War (1842) was a religious power struggle waged 1618-48, mainly on German soil. The symbol -30- as printer and telegrapher's code to indicate the last sheet or line of copy or a dispatch is recorded from 1895. In 20c. jargon of newspaper journalism, it came to be a traditional sign-off signal and slang word for "the end."
this (pron.) Look up this at Dictionary.com
Old English þis, neuter demonstrative pronoun and adjective (masc. þes, fem. þeos), probably from a North Sea Germanic pronoun *tha-si-, formed by combining the base *þa- (see that) with -s, which is probably identical with Old English se "the" (representing here "a specific thing"), or with Old English seo, imperative of see (v.) "to behold." Compare Old Saxon these, Old Frisian this, Old Norse þessi, Middle Dutch dese, Dutch deze, Old High German deser, German dieser.

Once fully inflected, with 10 distinct forms (see table below); the oblique cases and other genders gradually fell away by 15c. The Old English plural was þæs (nominative and accusative), which in Northern Middle English became thas, and in Midlands and Southern England became thos. The Southern form began to be used late 13c. as the plural of that (replacing Middle English tho, from Old English þa) and acquired an -e (apparently from the influence of Middle English adjective plurals in -e; compare alle from all, summe from sum "some"), emerging early 14c. as modern those.

About 1175 thes (probably a variant of Old English þæs) began to be used as the plural of this, and by 1200 it had taken the form these, the final -e acquired via the same mechanism that gave one to those.

Masc.Fem.Neut.Plural
Nom.þesþeosþisþas
Acc.þisneþasþisþas
Gen.þissesþisseþissesþissa
Dat.þissumþisseþissumþissum
Inst.þysþisseþysþissum
thistle (n.) Look up thistle at Dictionary.com
prickly herbaceous plant, Old English þistel, from Proto-Germanic *thistilaz (cognates: Old Saxon thistil, Old High German distil, German Distel, Old Norse þistell, Danish tidsel), of uncertain origin; perhaps from an extended form of PIE root *steig- "to prick, stick, pierce." Emblematic of Scotland since 15c.
thither (adv.) Look up thither at Dictionary.com
Old English þider "to or toward that place," altered (by influence of its opposite hider) from earlier þæder "to that place," from Proto-Germanic *thadra- (cognates: Old Norse þaðra "there," Gothic þaþro "thence"), from PIE pronominal root *to- (see that) + PIE suffix denoting motion toward (compare Gothic -dre, Sanskrit -tra). The medial -th- developed early 14c. but was rare before early 16c. (compare gather, murder, burden).
thixotropy (adj.) Look up thixotropy at Dictionary.com
1927, coined in German from Greek thixis "touching" (related to thinganein "to touch," from PIE root *dheigh- "to form, build; see dough) + trope "turning" (see trope (n.)). Related: Thixotropic.
tho (conj.) Look up tho at Dictionary.com
in modern use, an abbreviated spelling of though.
thole (v.) Look up thole at Dictionary.com
"to be subjected to or exposed to, to endure without complaint," now Scottish and Northern English dialect, from Old English þolian "to suffer, endure, undergo; remain, survive; to lose, lack, forfeit," from Proto-Germanic stem *thul- (cognates: Old Saxon tholon, Old High German dolon, Old Norse þola, Gothic þulan "to suffer," German geduld "patience"), from PIE *tele- "to bear, carry" (see extol).
thole (n.) Look up thole at Dictionary.com
"peg," from Old English þoll "oar-pin," from Proto-Germanic *thulnaz (cognates: Old Norse þollr, Middle Low German dolle, East Frisian dolle, Dutch dol), of unknown origin. No record of the word in English from c. 1000 to mid-15c.
Thomas Look up Thomas at Dictionary.com
masc. proper name, from Greek Thomas, of Aramaic origin and said to mean "a twin" (John's gospel refers to Thomas as ho legomenos didymos "called the twin;" compare Syriac toma "twin," Arabic tau'am "twin"). Before the Conquest, found only as the name of a priest, but after 1066, one of the most common given names in English. Also see Tom, Tommy. Doubting Thomas is from John xx:25; A Thomist (1530s, from Medieval Latin Thomista, mid-14c.) is a follower of 13c. scholastic theologian St. Thomas Aquinas.
Thompson (n.) Look up Thompson at Dictionary.com
type of sub-machine gun, 1919, named for U.S. Gen. John T. Thompson (1860-1940), who conceived it and whose company financed it. Familiarly Tommy gun by 1929.
thong (n.) Look up thong at Dictionary.com
Old English þwong, þwang "narrow strip of leather" (used as a cord, band, strip, etc.), from Proto-Germanic *thwang- (cognates: Old Norse þvengr), from PIE root *twengh- "to press in on, to restrain" (cognates: Old English twengan "to pinch, squeeze"). As a kind of sandal, first attested 1965; as a kind of bikini briefs, 1990.
Thor Look up Thor at Dictionary.com
Odin's eldest son, strongest of the gods though not the wisest, c.1020, from Old Norse Þorr, literally "thunder," from *þunroz, related to Old English þunor (see thunder (n.)). His weapon was the hammer mjölnir ("crusher").
thoracic (adj.) Look up thoracic at Dictionary.com
1650s, from stem of thorax + -ic, or else from Medieval Latin thoracicus.
thoracotomy (n.) Look up thoracotomy at Dictionary.com
1890, from comb. form of thorax + -ectomy.
thoral (adj.) Look up thoral at Dictionary.com
1690s, from Latin torus "couch, marriage bed, stuffed cushion" + -al (1).
thorax (n.) Look up thorax at Dictionary.com
"chest of the body," late 14c., from Latin thorax "the breast, chest; breastplate," from Greek thorax (genitive thorakos) "breastplate, chest," of unknown origin.
Thorazine (n.) Look up Thorazine at Dictionary.com
central nervous system depressant, 1954, proprietary name (Smith, Kline & French) formed from a rearrangement of various elements in the full chemical name.
thorium (n.) Look up thorium at Dictionary.com
rare metallic element, 1832, Modern Latin, named by its discoverer, Swedish chemist Jöns Jakob Berzelius (1779-1848) from thorite (silicate of thorium), the name of a mineral found in Norway from which it was extracted (which Berzelius also had named, as thoria, in 1828), and named in honor of the Scandinavian god Thor. With metallic element ending -ium.
thorn (n.) Look up thorn at Dictionary.com
Old English þorn "sharp point on a stem or branch," earlier "thorny tree or plant," from Proto-Germanic *thurnuz (cognates: Old Saxon, Old Frisian thorn, Dutch doorn, Old High German dorn, German Dorn, Old Norse þorn, Gothic þaurnus), from PIE *trnus (cognates: Old Church Slavonic trunu "thorn," Sanskrit trnam "blade of grass," Greek ternax "stalk of the cactus," Irish trainin "blade of grass"), from *(s)ter-n- "thorny plant," from root *ster- (1) "stiff" (see stark).

Figurative sense of "anything which causes pain" is recorded from early 13c. (thorn in the flesh is from II Cor. xii:7). Also an Anglo-Saxon and Icelandic runic letter (þ), named for the word of which it was the initial (see -th-).
thorny (adj.) Look up thorny at Dictionary.com
Old English þornig; see thorn + -y (2). Figurative sense is attested from mid-14c. Related: Thorniness. Similar formation in Dutch doornig, German dornig.