Online Etymology Dictionary

one (n.): c. 1200, from Old English an (adjective, pronoun, noun) "one," from Proto-Germanic *ainaz (source also of Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE *oi-no- "one, unique" (source also of Greek oinos "ace (on dice);" Latin unus "one;" Old Persian aivam; Old Church Slavonic -inu, ino-; Lithuanian vienas; Old Irish oin; Breton un "one").

Originally pronounced as it still is in only, and in dialectal good 'un, young 'un, etc.; the now-standard pronunciation "wun" began c. 14c. in southwest and west England (Tyndale, a Gloucester man, spells it won in his Bible translation), and it began to be general 18c. Use as indefinite pronoun influenced by unrelated French on and Latin homo.

One and only "sweetheart" is from 1906. One of those things "unpredictable occurrence" is from 1934. Slang one-arm bandit "a type of slot machine" is recorded by 1938. One-night stand is 1880 in performance sense; 1963 in sexual sense. One of the boys "ordinary amiable fellow" is from 1893. One-track mind is from 1927. Drinking expression one for the road is from 1950 (as a song title).
one-horse (adj.): "small-scale, petty" 1853, American English, colloquial, in reference to towns so small they only had one horse.
one-liner (n.): "short joke, witty remark," 1969, from one + line.
one-of-a-kind: adjectival phrase attested from 1961.
one-off (n.): "single example of a manufactured product," 1934, from one + off. Later given figurative extension.
one-shot (adj.): 1907, "achieved in a single attempt" (original reference is to golf), from one + shot (n.). Meaning "happening or of use only once" is from 1937.
one-sided (adj.): 1833, "dealing with one side of a question or dispute," from one + side (n.). Related: One-sidedly; one-sidedness.
one-upsmanship (n.): 1952, from noun phrase one up "scoring one more point than one's opponent" (1919).
one-way (adj.): 1906, in reference to travel tickets; 1914 in reference to streets; 1940 in reference to windows, mirrors, etc.; from one + way (n.).
Oneida: Iroquois tribe of upper N.Y. state, who later moved in part to Wisconsin, 1666, named for its principal settlement, from Oneida onenyote', literally "erected stone," containing -neny- "stone" and -ot- "to stand."
oneiric (adj.): 1859, from Greek oneiros "a dream" (see oneiro-) + -ic.
oneiro-: before vowels oneir-, word-forming element meaning "dream," from Greek oneiros "a dream," from PIE *oner- "dream."
oneirocritic (n.): "a judge or interpreter of dreams," 1650s from Greek oneirokritikos "pertaining to the interpretation of dreams," from oneirokrites "interpreter of dreams," from oneiros "a dream" (see oneiro-) + krites "discerner, judge" (see critic).
oneirocritical (adj.): 1580s, from oneiro- + critic + -al (1).
oneiromancy (n.): 1650s; see oneiro- + -mancy. Greek had oneiromantis "an interpreter of dreams." Related: oneiromantic.
oneness (n.): 1590s, from one + -ness. A re-formation of Middle English onnesse, which vanished by 13c.
onerous (adj.): late 14c., from Old French onereus, honereus (14c., Modern French onéreux) and directly from Latin onerosus, from onus (genitive oneris) "burden" (see onus).
oneself: 1540s, one's self. Hyphenated 18c.; written as one word from c. 1827, on model of himself, itself, etc.
ongoing (adj.): also on-going, 1877, from on + going (see go).
onion (n.): early 12c., from Anglo-French union, Old French oignon "onion" (formerly also oingnon), and directly from Latin unionem (nominative unio), colloquial rustic Roman for "a kind of onion," also "pearl" (via notion of a string of onions), literally "one, unity;" sense connection is the successive layers of an onion, in contrast with garlic or cloves.

Old English had ynne (in ynne-leac), from the same Latin source, which also produced Irish inniun, Welsh wynwyn and similar words in Germanic. In Dutch, the ending in -n was mistaken for a plural inflection and new singular ui formed. The usual Indo-European name is represented by Greek kromion, Irish crem, Welsh craf, Old English hramsa, Lithuanian kremuse.

The usual Latin word was cepa, a loan from an unknown language; it is the source of Old French cive, Old English cipe, and, via Late Latin diminutive cepulla, Italian cipolla, Spanish cebolla, Polish cebula. German Zwiebel also is from this source, but altered by folk etymology in Old High German (zwibolla) from words for "two" and "ball." Onion ring is attested from 1952.

Onion dome attested from 1956; onion grass from 1883; onion skin as a type of paper from 1892. Onions, the surname, is attested from mid-12c. (Ennian), from Old Welsh Enniaun, ultimately from Latin Annianus, which was associated with Welsh einion "anvil."
oniony (adj.): 1838, from onion + -y (2). Related: Onioniness.
online (adj.): in reference to computers, "directly connected to a peripheral device," 1950 (originally as on-line).
onlooker (n.): c. 1600, from on + agent noun from look (v.).
only (adj.): Old English ænlic, anlic "only, unique, solitary," literally "one-like," from an "one" (see one) + -lic "-like" (see -ly (1)). Use as an adverb and conjunction developed in Middle English. Distinction of only and alone (now usually in reference to emotional states) is unusual; in many languages the same word serves for both. German also has a distinction in allein/einzig. Phrase only-begotten (mid-15c.) is biblical, translating Latin unigenitus, Greek monogenes. The Old English form was ancenned.
onnagata (n.): in Kabuki and similar drama, a man who plays female roles, 1901, from Japanese, from onna "woman" + kata "figure."
onomastic (adj.): 1716, from French onomastique (17c.), from Greek onomastikos "of or belonging to naming," from onomastos "named," verbal adjective of onomazein "to name," from onoma "name" (see name).
onomastics (n.): "scientific study of names and naming," 1936, from onomastic; also see -ics.
onomatopoeia (n.): 1570s, from Late Latin onomatopoeia, from Greek onomatopoiia "the making of a name or word" (in imitation of a sound associated with the thing being named), from onomatopoios, from onoma (genitive onomatos) "word, name" (see name (n.)) + a derivative of poiein "compose, make" (see poet). Related: Onomatopoeic; onomatopoeial.
onomatopoeic (adj.): 1860, from French onomatopoéique or else from onomatopoeia + -ic.
Onondaga: tribe in the Iroquois Confederacy, 1684, named for its principal settlement, from Onondaga onontake, literally "on the hill."
onset (n.): 1530s, "attack, assault," from on + set (n.); compare verbal phrase to set (something) on (someone). Weaker sense of "beginning, start" first recorded 1560s. Figurative use in reference to a calamity, disease, etc. is from 1580s.
onslaught (n.): 1620s, anslaight, somehow from or on analogy of Dutch aanslag "attack," from Middle Dutch aenslach, from aen "on" (see on) + slach "blow," related to slaen "slay." Spelling influenced by obsolete (since c. 1400) English slaught (n.) "slaughter," from Old English sleaht (see slaughter (n.)). No record of its use in 18c.; apparently revived by Scott.
Ontario: from Mohawk (Iroquoian) ontari:io "beautiful lake" or "great lake," from /-qtar-/ "lake, river." Related: Ontarian.
ontic (adj.): 1949, from onto- + -ic.
onto (prep.): 1580s, as on to, from on + to. Appeared much later than parallel into. As a closed compound (on analogy of into), first recorded 1715.
onto-: word-forming element meaning "a being, individual; being, existence," from Greek onto-, from stem of on (genitive ontos) "being," neuter present participle of einai "to be" (see essence).
ontogeny (n.): "development of an individual," 1872, from onto- + -geny. Related: Ontogenic; ontogenesis.
ontological (adj.): 1782, from ontology + -ical. Related: Ontologically
ontology (n.): "metaphysical science or study of being," 1660s (Gideon Harvey), from Modern Latin ontologia (c. 1600), from onto- + -logy.
onus (n.): 1640s, from Latin onus "load, burden," figuratively "tax, expense; trouble, difficulty," from PIE *en-es- "burden" (source of Sanskrit anah "cart, wagon"). Hence legal Latin onus probandi (1722), literally "burden of proving."
onward (adv.): late 14c., from on + -ward. The form onwards, with adverbial genitive -s-, is attested from c. 1600.
onwards (adv.): see onward.
onymous (adj.): 1775, coined to provide an opposite to anonymous. Related: Onymously.
onyx (n.): mid-13c., from Old French oniche "onyx" (12c.), and directly from Latin onyx (genitive onychis), from Greek onyx "onyx-stone," originally "claw, fingernail" (see nail (n.)). So called because the mineral's color sometimes resembles that of a fingernail, pink with white streaks.
oo-: word-forming element meaning "egg, eggs," from Greek oon "egg," cognate with Latin ovum, Old Norse egg (see egg (n.)).
oocyte (n.): 1895, from oo- + -cyte (see cyto-).
oodles (n.): "lots," 1867, American English (originally in a Texas context), perhaps from the caboodle in kit and caboodle.
oogenesis (n.): "formation of the ovum," 1892, from oo- + -genesis "birth, origin, creation."
ooh: exclamation of pain, surprise, wonder, etc., 1916. Combined with aah from 1953. Ooh-la-la, exclamation of surprise or appreciation, is attested 1924, from French and suggestive of the supposed raciness of the French.
oolite (n.): "rock consisting of fine grains of carbonate of lime," 1785, from Modern Latin oolites, from oo-, comb. form of Greek oon "egg" (cognate with Old English æg, see egg (n.)) + lithos "stone" (see litho-). So called because the rock resembles the roe of fish.