o Look up o at Dictionary.com
interjection of fear, surprise, admiration, etc.; see oh.
O Look up O at Dictionary.com
blood type, 1926, originally "zero," denoting absence of A and B agglutinogens.
O' Look up O' at Dictionary.com
as a prefix in Irish names, from Irish ó, ua (Old Irish au) "descendant."
o'clock (adj.) Look up o'clock at Dictionary.com
c. 1720, abbreviation of of the clock (1640s), from Middle English of the clokke (late 14c.). Use of clock hand positions to describe vector directions or angles is from late 18c.
o'er Look up o'er at Dictionary.com
poetic contraction of over.
O.D. Look up O.D. at Dictionary.com
abbreviation of overdose, attested from 1960.
oaf (n.) Look up oaf at Dictionary.com
1620s, auf, oph (modern form from 1630s), "a changeling; a foolish child left by the fairies" [Johnson], from a Scandinavian source such as Norwegian alfr "silly person," in Old Norse "elf" (see elf). Hence, "a misbegotten, deformed idiot." Until recently, some dictionaries still gave the plural as oaves.
oafish (adj.) Look up oafish at Dictionary.com
1610s, from oaf + -ish. Related: Oafishly; oafishness.
oak (n.) Look up oak at Dictionary.com
Old English ac "oak tree," from Proto-Germanic *aiks (source also of Old Norse eik, Old Saxon and Old Frisian ek, Middle Dutch eike, Dutch eik, Old High German eih, German Eiche), of uncertain origin with no certain cognates outside Germanic.

The usual Indo-European base for "oak" (*deru-) has become Modern English tree (n.); likewise in Greek and Celtic words for "oak" are from the Indo-European root for "tree," probably reflecting the importance of the oak to ancient Indo-Europeans. The Old Norse form was eik, but as there were no oaks in Iceland the word came to be used there for "tree" in general. Used in Biblical translations to render Hebrew elah (probably usually "terebinth tree") and four other words.
oaken (adj.) Look up oaken at Dictionary.com
"made of, or consisting of, oak," late 12c., from oak + -en (2).
oakum (n.) Look up oakum at Dictionary.com
"loose fiber obtained from taking apart old hemp ropes," early 15c., from Old English acumba "tow, oakum, flax fibers separated by combing," literally "what is combed out," from Proto-Germanic *us-kambon (source of Old High German achambi); first element cognate with Old English a- "away, out, off;" second element from stem of cemban "to comb," from camb "a comb;" from PIE *gembh- "tooth, nail" (see comb (n.)).
oar (n.) Look up oar at Dictionary.com
Old English ar "oar," from Proto-Germanic *airo (source also of Old Norse ar, Danish aare, Swedish åra), of unknown origin; perhaps related to Latin remus "oar," Greek eretes "rower," eretmos "oar."
oarlock (n.) Look up oarlock at Dictionary.com
mid-14c., from oar + lock (n.1).
oasis (n.) Look up oasis at Dictionary.com
1610s, from French oasis (18c.) and directly from Late Latin oasis, from Greek oasis, probably from Hamitic (compare Coptic wahe, ouahe "oasis," properly "dwelling place," from ouih "dwell"). The same Egyptian source produced Arabic wahah.
oat (n.) Look up oat at Dictionary.com
Old English ate (plural atan) "grain of the oat plant, wild oats," of uncertain origin, possibly from Old Norse eitill "nodule," denoting a single grain, of unknown origin. The English word has cognates in Frisian and some Dutch dialects. Famously defined by Johnson as, "A grain, which in England is generally given to horses, but in Scotland supports the people."

The usual Germanic name is derived from Proto-Germanic *khabran (source also of Old Norse hafri, Dutch haver, source of haversack). Wild oats, "crop that one will regret sowing," is first attested 1560s, in reference to the folly of sowing these instead of good grain.
That wilfull and vnruly age, which lacketh rypenes and discretion, and (as wee saye) hath not sowed all theyr wyeld Oates. [Thomas Newton, "Lemnie's Touchstone of complexions," 1576]

Fred: I still want to sow some wild oats!
Lamont: At your age, you don't have no wild oats, you got shredded wheat.
["Sanford and Son"]
Hence, to feel (one's) oats "be lively," 1831, originally American English.
oater (n.) Look up oater at Dictionary.com
Western film, "horse opera," 1946, from oat, as the typical food of horses.
oath (n.) Look up oath at Dictionary.com
Old English "oath, judicial swearing, solemn appeal to deity in witness of truth or a promise," from Proto-Germanic *aithaz (source also of Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (source also of Old Irish oeth "oath"). Common to Celtic and Germanic, possibly a loan-word from one to the other, but the history is obscure. In reference to careless invocations of divinity, from late 12c.
oatmeal (n.) Look up oatmeal at Dictionary.com
late 14c., from oat + Middle English mele (see meal (n.2)).
ob- Look up ob- at Dictionary.com
word-forming element meaning "toward, against, across, down," also used as an intensive, from Latin ob (prep.) "in front of, before; in the way of; with regard to, because of," from PIE root *epi, also *opi "near, against" (see epi-).
Obadiah Look up Obadiah at Dictionary.com
masc. proper name, fourth of the Twelve Prophets of the Old Testament, from Hebrew Obhadyah, literally "servant of the Lord," from abhadh "he served, worshipped," related to Arabic 'abada "he served," 'abd "slave, worshipper."
obbligato (adj.) Look up obbligato at Dictionary.com
musical instruction, 1724, from Italian obbligato, literally "obligated," from Latin obligatus, past participle of obligare "to bind" (see oblige). In reference to a necessary accompaniment by a single instrument.
obduracy (n.) Look up obduracy at Dictionary.com
"stubbornness," 1590s, from obdurate + -cy.
obdurate (adj.) Look up obdurate at Dictionary.com
mid-15c., "stubborn; hardened," from Latin obduratus "hardened," past participle of obdurare "be hard, hold out, persist, endure," from ob "against" (see ob-) + durare "harden, render hard," from durus "hard" (see endure). Related: Obdurately.
obduration (n.) Look up obduration at Dictionary.com
c. 1400, "hard-heartedness," from Latin obdurationem (nominative obduratio), noun of state from past participle stem of obdurare (see obdurate).
obeah (n.) Look up obeah at Dictionary.com
"sorcery, witchcraft" among blacks in Africa and the W.Indies, 1760, from a West African word, such as Efik (southern Nigeria) ubio "a thing or mixture left as a charm to cause sickness or death," Twi ebayifo "witch, wizard, sorcerer."
obedience (n.) Look up obedience at Dictionary.com
c. 1200, "submission to a higher power or authority," from Old French obedience "obedience, submission" (12c.) and directly from Latin oboedientia "obedience," noun of quality from oboedientem (nominative oboediens); see obedient. In reference to dog training from 1930.
obedient (adj.) Look up obedient at Dictionary.com
c. 1200, from Old French obedient "obedient" (11c.), from Latin oboedientem (nominative oboediens), present participle of oboedire "to obey" (see obey). Related: Obediently.
obeisance (n.) Look up obeisance at Dictionary.com
late 14c., "act or fact of obeying," from Old French obeissance "obedience, service, feudal duty" (13c.), from obeissant, present participle of obeir "obey," from Latin oboedire (see obey). Sense in English altered late 14c. to "bending or prostration of the body as a gesture of submission or respect" by confusion with abaisance. Related: Obeisant.
obelisk (n.) Look up obelisk at Dictionary.com
"rectangular stone column tapering at the top," 1560s, from Middle French obélisque (16c.) and directly from Latin obeliscus "obelisk, small spit," from Greek obeliskos "small spit, obelisk, leg of a compass," diminutive of obelos "a spit, pointed pillar, needle." Related: Obeliskine.
Oberon Look up Oberon at Dictionary.com
king of the faeries and husband of Titania in medieval lore, from French Obéron, from Old French Auberon, perhaps from a Germanic source related to elf.
obese (adj.) Look up obese at Dictionary.com
1650s, back-formation from obesity and in part from Latin obesus "fat, stout, plump," past participle of obedere "that has eaten itself fat" (see obesity). According to OED, "Rare before 19th c." Related: Obeseness. Latin obesus was translated in Old English as oferfæt "overfat."
obesity (n.) Look up obesity at Dictionary.com
1610s, from French obésité and directly from Latin obesitas "fatness, corpulence," from obesus "that has eaten itself fat," past participle of obedere "to eat all over, devour," from ob "over" (see ob-) + edere "eat" (see edible).
obey (v.) Look up obey at Dictionary.com
late 13c., from Old French obeir "obey, be obedient, do one's duty" (12c.), from Latin obedire, oboedire "obey, be subject, serve; pay attention to, give ear," literally "listen to," from ob "to" (see ob-) + audire "listen, hear" (see audience). Same sense development is in cognate Old English hiersumnian. Related: Obeyed; obeying.
obfuscate (v.) Look up obfuscate at Dictionary.com
1530s, from Latin obfuscatus, past participle of obfuscare "to darken," from ob "in front of, before" (see ob-) + fuscare "to make dark," from fuscus "dark" (see dusk). Related: Obfuscated; obfuscating.
obfuscation (n.) Look up obfuscation at Dictionary.com
early 15c., originally medical, "the darkening of a sore," from Latin obfuscationem (nominative obfuscatio), noun of action from past participle stem of obfuscare (see obfuscate).
Obie Look up Obie at Dictionary.com
one of the annual awards given to off-Broadway theater, 1967, from O.B. as the abbreviation of Off-Broadway.
obit (n.) Look up obit at Dictionary.com
late 14c., "death," from Middle French obit or directly from Latin obitus "death," noun use of past participle of obire "to die," literally "to go toward" (see obituary). In modern usage (since 1874) it is usually a clipped form of obituary, though it had the same meaning of "published death notice" 15c.-17c. The scholarly abbreviation ob. with date is from Latin obiit "(he) died," third person singular of obire.
obiter (adv.) Look up obiter at Dictionary.com
Latin, literally "by the way," from ob "in front of, toward" (see ob-) + iter "journey" (see ion). Klein's sources, however, say it is ob with the suffix -iter in analogy of circiter "about" from circa. Also see obituary.
obiter dictum Look up obiter dictum at Dictionary.com
"statement in passing," a judge's expression of opinion not regarded as binding or decisive, Latin, literally "something said incidentally;" see obiter + dictum.
obituary (n.) Look up obituary at Dictionary.com
1706, "register of deaths," from Medieval Latin obituarius "a record of the death of a person," literally "pertaining to death," from Latin obitus "departure, a going to meet, encounter" (a euphemism for "death"), from stem of obire "go toward, go to meet" (as in mortem obire "meet death"), from ob "toward" (see ob-) + ire "to go" (see ion). Meaning "record or announcement of a death, especially in a newspaper, and including a brief biographical sketch" is from 1738. As an adjective from 1828. A similar euphemism is in Old English cognate forðfaran "to die," literally "to go forth;" utsið "death," literally "going out, departure."
object (v.) Look up object at Dictionary.com
c. 1400, "to bring forward in opposition," from Old French objecter and directly from Latin obiectus, past participle of obiectare "to cite as grounds for disapproval, set against, oppose," literally "to put or throw before or against," frequentative of obicere (see object (n.)). Related: Objected; objecting.
object (n.) Look up object at Dictionary.com
late 14c., "tangible thing, something perceived or presented to the senses," from Medieval Latin objectum "thing put before" (the mind or sight), noun use of neuter of Latin obiectus "lying before, opposite" (as a noun in classical Latin, "charges, accusations"), past participle of obicere "to present, oppose, cast in the way of," from ob "in front of, towards, against" (see ob-) + iacere "to throw" (see jet (v.)). Sense of "thing aimed at" is late 14c. No object "not a thing regarded as important" is from 1782. As an adjective, "presented to the senses," from late 14c. Object lesson "instruction conveyed by examination of a material object" is from 1831.
objectification (n.) Look up objectification at Dictionary.com
1860, noun of action from objectify.
objectify (v.) Look up objectify at Dictionary.com
1838, from Medieval Latin objectum (see object (n.)) + -fy. Related: Objectified; objectifying.
objection (n.) Look up objection at Dictionary.com
late 14c., from Old French objeccion "reply, retort" (12c.) and directly from Late Latin obiectionem (nominative obiectio), "a throwing or putting before," noun of action from past participle stem of Latin obicere "to oppose" (see object (n.)).
objectionable (adj.) Look up objectionable at Dictionary.com
1781, from objection + -able. Related: Objectionably.
objective (adj.) Look up objective at Dictionary.com
1610s, originally in the philosophical sense of "considered in relation to its object" (opposite of subjective), formed on pattern of Medieval Latin objectivus, from objectum "object" (see object (n.)) + -ive. Meaning "impersonal, unbiased" is first found 1855, influenced by German objektiv. Related: Objectively.
objective (n.) Look up objective at Dictionary.com
1738, "something objective to the mind," from objective (adj.). Meaning "goal, aim" (1881) is from military term objective point (1852), reflecting a sense evolution in French.
objectivism (n.) Look up objectivism at Dictionary.com
1854 in philosophical sense, "the doctrine that knowledge is based on objective reality," from objective (adj.) + -ism.
objectivity (n.) Look up objectivity at Dictionary.com
1803, from Medieval Latin objectivus, from Latin objectus (see object (n.)) + -ity.