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 pronomial 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 chemical 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.
thoro Look up thoro at Dictionary.com
abbreviated spelling of thorough.
thorough (adj.) Look up thorough at Dictionary.com
c.1300, adjectival use of Old English þuruh (adv.) "from end to end, from side to side," stressed variant of þurh (adv., prep.); see through. Related: thoroughly; thoroughness.
thoroughbred (adj.) Look up thoroughbred at Dictionary.com
1701, of persons, "thoroughly accomplished," from thorough + past tense of breed. In reference to horses, "of pure breed or stock," from 1796; the noun is first recorded 1842.
thoroughfare (n.) Look up thoroughfare at Dictionary.com
late 14c., "passage or way through," from thorough (before it had differentiated from through) + fare (n.).
thoroughgoing (adj.) Look up thoroughgoing at Dictionary.com
1800, from thorough + going.
thorp (n.) Look up thorp at Dictionary.com
Old English ðorp "village, hamlet, farm, estate," reinforced by Old Norse ðorp, both from Proto-Germanic *thurpa- (cognates: Old Frisian thorp, Frisian terp, Middle Dutch, Dutch dorp, German dorf "village," Gothic þaurp "estate, land, field"), probably from PIE root *treb- "dwelling" (see tavern). Preserved in place names ending in -thorp, -thrup.
those (pron.) Look up those at Dictionary.com
c.1300, Midlands and southern variant of Old English þas, nominative and accusative plural of þes, þeos "this" (see this). A collateral form of these, now used as the plural of that.
Thoth Look up Thoth at Dictionary.com
ancient Egyptian god of wisdom and magic, hieroglyphics, and the reckoning of time, from Latin, from Greek Thoth, from Egyptian Tehuti. Usually represented as a human figure with the head of an ibis. By the Greeks, assimilated to their Hermes.
thou (pron.) Look up thou at Dictionary.com
2nd nominative singular personal pronoun, Old English þu, from Proto-Germanic *thu (cognates: Old Frisian thu, Middle Dutch and Middle Low German du, Old High German and German du, Old Norse þu, Gothic þu), from PIE *tu-, second person singular pronoun (cognates: Latin tu, Irish tu, Welsh ti, Greek su, Lithuanian tu, Old Church Slavonic ty, Sanskrit twa-m).

Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. early Quakers). The plural at first was used in addressing superior individuals, later also (to err on the side of propriety) strangers, and ultimately all equals. By c.1450 the use of thou to address inferiors gave it a tinge of insult unless addressed by parents to children, or intimates to one another. Hence the verb meaning "to use 'thou' to a person" (mid-15c.).
Avaunt, caitiff, dost thou thou me! I am come of good kin, I tell thee! ["Hickscorner," c.1530]
A brief history of the second person pronoun in English can be found here.
though (adv., conj.) Look up though at Dictionary.com
c.1200, from Old English þeah "though, although, even if, however, nevertheless, although, still, yet;" and in part from Old Norse þo "though," both from Proto-Germanic *thaukh (cognates: Gothic þauh, Old Frisian thach, Middle Dutch, Dutch doch, Old High German doh, German doch), from PIE demonstrative pronoun *to- (see that). The evolution of the terminal sound did not follow laugh, tough, etc., though a tendency to end the word in "f" existed c.1300-1750 and persists in dialects.
thought (n.) Look up thought at Dictionary.com
Old English þoht, geþoht "process of thinking, a thought; compassion," from stem of þencan "to conceive of in the mind, consider" (see think). Cognate with the second element in German Gedächtnis "memory," Andacht "attention, devotion," Bedacht "consideration, deliberation." Second thought "later consideration" is recorded from 1640s. Thought-crime is from "Nineteen Eighty-Four" (1949); thought police is attested from 1945, originally in reference to war-time Japanese Special Higher Police (Tokubetsu Koto Keisatsu).
thoughtful (adj.) Look up thoughtful at Dictionary.com
c.1200, "contemplative, occupied with thought," from thought + -ful. Also in Middle English, "prudent; moody, anxious." Meaning "showing consideration for others" is from 1851 (compare thoughtless.) Related: Thoughtfully; thoughtfulness.
thoughtless (adj.) Look up thoughtless at Dictionary.com
1610s, "heedless, imprudent," from thought + -less. Meaning "inconsiderate of others" is from 1794. Related: Thoughtlessly; thoughtlessness.
thousand (adj.) Look up thousand at Dictionary.com
Old English þusend, from Proto-Germanic *thusundi (cognates: Old Frisian thusend, Dutch duizend, Old High German dusunt, German tausend, Old Norse þusund, Gothic þusundi).

Related to words in Balto-Slavic (Lithuanian tukstantis, Old Church Slavonic tysashta, Polish tysiąc, Russian tysiacha, Czech tisic), and probably ultimately a compound with indefinite meaning "great multitude, several hundred," literally "swollen-hundred," with first element from PIE root *teue- (2) "to swell" (see thigh).

Used to translate Greek khilias, Latin mille, hence the refinement into the precise modern meaning. There was no general Indo-European word for "thousand." Slang shortening thou first recorded 1867. Thousand island dressing (1916) presumably is named for the region of New York on the St. Lawrence River.
thousandth (adj.) Look up thousandth at Dictionary.com
1550s, from thousand + -th (1).
Thrace Look up Thrace at Dictionary.com
Greek Thrake, named for the people who inhabited it, whose name is of unknown origin, perhaps Semitic. Related: Thracian.
thraldom (n.) Look up thraldom at Dictionary.com
also thralldom, c.1200; see thrall + -dom.
thrall (n.) Look up thrall at Dictionary.com
late Old English þræl "bondman, serf, slave," from a Scandinavian source akin to Old Norse þræll "slave, servant," figuratively "wretch, scoundrel," probably from Proto-Germanic *thrakhilaz, literally "runner," from root *threh- "to run" (cognates: Old High German dregil "servant," properly "runner;" Old English þrægan, Gothic þragjan "to run"). Meaning "condition of servitude" is from early 14c.
thrash (v.) Look up thrash at Dictionary.com
1580s, "to separate grains from wheat, etc., by beating," dialectal variant of threshen (see thresh). Sense of "beat (someone) with (or as if with) a flail" is first recorded 1620s. Meaning "to make wild movements like those of a flail or whip" is attested from 1846. Related: Thrashed; thrashing. As a noun from 1660s, "threshing tool;" 1840s as "a beating;" 1982 as the name for a type of fast heavy metal music.
thread (n.) Look up thread at Dictionary.com
Old English þræd "fine cord, especially when twisted" (related to þrawan "to twist"), from Proto-Germanic *thredu- "twisted yarn" (cognates: Old Saxon thrad, Old Frisian thred, Middle Dutch draet, Dutch draad, Old High German drat, German Draht, Old Norse þraðr), literally "twisted," from suffixed form of PIE root *tere- (1) "to rub, rub by turning, turn" (see throw (v.)). Meaning "spiral ridge of a screw" is from 1670s. Threads, slang for "clothes" is 1926, American English.
thread (v.) Look up thread at Dictionary.com
"to put thread through a needle," mid-14c., from thread (n.); in reference to film cameras from 1913. The dancing move called thread the needle is attested from 1844. Related: Threaded; threading.
threadbare (adj.) Look up threadbare at Dictionary.com
late 14c., from thread (n.) + bare. The notion is of "having the nap worn off," leaving bare the threads.
threat (n.) Look up threat at Dictionary.com
Old English þreat "crowd, troop," also "oppression, coercion, menace," related to þreotan "to trouble, weary," from Proto-Germanic *thrautam (cognates: Dutch verdreiten, German verdrießen "to vex"), from PIE *treud- "to push, press squeeze" (cognates: Latin trudere "to press, thrust," Old Church Slavonic trudu "oppression," Middle Irish trott "quarrel, conflict," Middle Welsh cythrud "torture, torment, afflict"). Sense of "conditional declaration of hostile intention" was in Old English.
threaten (v.) Look up threaten at Dictionary.com
late 13c., "attempt to influence by menacing," from Old English þreatnian "to threaten" (see threat). Related: Threatened. Threatening in the sense of "portending no good" is recorded from 1520s.
three (adj.) Look up three at Dictionary.com
Old English þreo, fem. and neuter (masc. þri, þrie), from Proto-Germanic *thrijiz (cognates: Old Saxon thria, Old Frisian thre, Middle Dutch and Dutch drie, Old High German dri, German drei, Old Norse þrir, Danish tre), from nominative plural of PIE root *trei- "three" (cognates: Sanskrit trayas, Avestan thri, Greek treis, Latin tres, Lithuanian trys, Old Church Slavonic trye, Irisn and Welsh tri "three").

3-D first attested 1952, abbreviation of three-dimensional (1878). Three-piece suit is recorded from 1909. Three cheers for ______ is recorded from 1751. Three-martini lunch is attested from 1972. Three-ring circus first recorded 1898. Three-sixty "complete turnaround" is from 1927, originally among aviators, in reference to the number of degrees in a full circle. Three musketeers translates French les trois mousquetaires, title of the 1844 novel by Alexandre Dumas père.
Three Rs (n.) Look up Three Rs at Dictionary.com
1824; said to have been given as a toast by Sir William Curtis (1752-1829), a beloved lord mayor of London in the 1820s, who seems to have been a figure of fun to whom many mangled phrases were attributed. Among the toasts he is alleged to have given at public dinners were "The Female Ladies of London;" "The three C's--Cox, King, and Curtis;" and "The three R's--Reading, Writing, and Rithmetic."
It has been very much the fashion amongst a class of persons to attribute to Sir W. C. ... a vulgarity and ignorance of speech which are by no means consistent with his character and conduct. The worthy and hospitable baronet has a rapid mode of speech, but it is always correct ; and although some eccentricities are mixed up in his composition, he is highly honourable, and has been a very useful member of society, particularly to his London constituents. ["The Mirror," Jan. 29, 1825]
After listing some examples, the article continues:
It is, however, very certain, that at a city festival some years ago, having indulged very freely, he fell asleep, when some wag, choosing to consider him dead, wrote his epitaph, which was found next morning pinned to the baronet's dress coat:--

"Here lies the great Curtis,
Of London, Lord May'r:
He's left this here world,
And gone to that there."
threefold (adj.) Look up threefold at Dictionary.com
late Old English þrifeald; see three + -fold.
threesome (n.) Look up threesome at Dictionary.com
late 14c., from three + -some (2).
threnody (n.) Look up threnody at Dictionary.com
"song of lamentation," 1630s, from Greek threnodia "lamentation," from threnos "dirge, lament" + oide "ode" (see ode). Greek threnos probably is from PIE imitative root *dher- (3) "to drone, murmur, hum;" cognates: Old English dran "drone," Gothic drunjus "sound," Greek tenthrene "a kind of wasp."
thresh (v.) Look up thresh at Dictionary.com
Old English þrescan, þerscan, "to beat, sift grain by trampling or beating," from Proto-Germanic *threskan "to thresh," originally "to tread, to stamp noisily" (cognates: Middle Dutch derschen, Dutch dorschen, Old High German dreskan, German dreschen, Old Norse þreskja, Swedish tröska, Gothic þriskan), from PIE root *tere- (1) "to rub, turn" (see throw (v.)).

The basic notion is of men or oxen treading out wheat; later, with the advent of the flail, the word acquired its modern extended sense of "to knock, beat, strike." The original Germanic sense is suggested by the use of the word in Romanic languages that borrowed it, such as Italian trescare "to prance," Old French treschier "to dance," Spanish triscar "to stamp the feet."
thresher (n.) Look up thresher at Dictionary.com
late 14c., agent noun from thresh. The thresher shark (c.1600) so called for its long upper tail, which resembles a threshing tool.
threshold (n.) Look up threshold at Dictionary.com
Old English þrescold, þærscwold, þerxold, etc., "door-sill, point of entering," probably literally "something to tread upon," with first element related to Old English þrescan (see thresh), with its original sense of "tread, trample." Second element of unknown origin and much transformed in all the Germanic languages, suggesting its literal sense was lost even in ancient times. In English it probably has been altered to conform to hold, but the oft-repeated story that the threshold was a barrier placed at the doorway to hold the chaff flooring in the room is mere folk etymology. Cognates include Old Norse þreskjoldr, Swedish tröskel, Old High German driscufli, German dialectal drischaufel. Figurative use was in Old English.