Etymology
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an 
indefinite article before words beginning with vowels, 12c., from Old English an (with a long vowel) "one; lone," also used as a prefix meaning "single, lone" (as in anboren "only-begotten," anhorn "unicorn," anspræce "speaking as one"). See one for the divergence of that word from this. Also see a, of which this is the older, fuller form.

In other European languages, identity between the indefinite article and the word for "one" remains explicit (French un, German ein, etc.). Old English got by without indefinite articles: He was a good man in Old English was he wæs god man.

In texts of Shakespeare, etc., an as a word introducing a clause stating a condition or comparison conjunction is a reduced form of and in this now-archaic sense "if" (a usage first attested late 12c.), especially before it.
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only (adj.)

"single as regards number, class, or kind," Middle English onli, from Old English ænlic, anlic "only, unique, solitary," literally "one-like," from an "one" (see one) + -lic "-like" (see -ly (1)). Similar formation in Old Frisian einlik, Dutch eenlijk, Old High German einlih, Danish einlig. It preserves the old pronunciation of one. Related: Onliness.

Its use as an adverb ("alone, no other or others than; in but one manner; for but one purpose") and conjunction ("but, except") 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 word was ancenned. Only child is attested by 1700.

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*oi-no- 
Proto-Indo-European root meaning "one, unique."

It forms all or part of: a (1) indefinite article; alone; an; Angus; anon; atone; any; eleven; inch (n.1) "linear measure, one-twelfth of a foot;" lone; lonely; non-; none; null; once; one; ounce (n.1) unit of weight; quincunx; triune; unanimous; unary; une; uni-; Uniate; unilateral; uncial; unicorn; union; unique; unison; unite; unity; universal; universe; university; zollverein.

It is the hypothetical source of/evidence for its existence is provided by: Greek oinos "ace (on dice);" Latin unus "one;" Old Persian aivam; Old Church Slavonic -inu, ino-; Lithuanian vienas; Old Irish oin; Breton un "one;" Old English an, German ein, Gothic ains "one."
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once (adv.)

"one time only; at one time in the past, formerly," c. 1200, anes, basically an adverbial form of one with adverbial genitive -s. The Old English form was æne, but it was replaced by, or reshaped by analogy with, the genitive singular of the early Middle English form of one and the common addition of -es to adverbs at that time. The spelling changed as pronunciation shifted from two syllables to one after c. 1300; the -ce is to retain the breathy -s- (compare hence). The pronunciation change to "wuns" parallels that of one.

As an emphatic, meaning "once and for all," it is attested from c. 1300, but in modern U.S. this is a Pennsylvania German dialect formation. Meaning "in a past time" (but not necessarily just one time) is from mid-13c.

Never once "never at all" is from early 13c. Once in a while "sometimes" is by 1781. Once upon a time as the beginning of a story is recorded from 1590s, earlier once on a time (late 14c.). At once originally (early 13c.) meant "simultaneously," later "in one company" (c. 1300), and preserved the sense of "one" in the word; the phrase typically appeared as one word, atones; the modern meaning "immediately" is attested from 1530s. Once and for all "once as a final act" is from 1848, earlier once for all (late 15c.).

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eleven (adj., n.)

"1 more than ten; the number which is one more than ten; a symbol representing this number;" c. 1200, elleovene, from Old English enleofan, endleofan, literally "one left" (over ten), from Proto-Germanic *ainlif- (compare Old Saxon elleban, Old Frisian andlova, Dutch elf, Old High German einlif, German elf, Old Norse ellifu, Gothic ainlif), a compound of *ain "one" (see one) + from PIE root *leikw- "to leave."

FIREFLY: Give me a number from 1 to 10.
CHICOLINI: eleven!
FIREFLY: Right!
["Duck Soup"]

Viking survivors who escaped an Anglo-Saxon victory were daroþa laf "the leavings of spears," while hamora laf "the leavings of hammers" was an Old English kenning for "swords" (both from "The Battle of Brunanburh"). Twelve reflects the same formation. Outside Germanic the only instance of this formation is in Lithuanian, which uses -lika "left over" and continues the series to 19 (vienuo-lika "eleven," dvy-lika "twelve," try-lika "thirteen," keturio-lika "fourteen," etc.). Meaning "a team or side" in cricket or football is from 1743.

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oneirocritical (adj.)

"having the power of interpreting dreams," 1580s; see oinerocritic + -al (1).

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oneirocritic (n.)

"a judge or interpreter of dreams," 1650s from Greek oneirokritikos "of or pertaining to the interpretation of dreams," from oneirokritēs "interpreter of dreams," from oneiros "a dream" (see oneiro-) + kritēs "discerner, judge" (see critic).

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oneiromancy (n.)

"divination through dreams," 1650s; see oneiro- "dream" + -mancy "divination by means of." Greek had oneiromantis "an interpreter of dreams." Related: oneiromantic.

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Oneida 

Iroquois people of upper N.Y. state (they later moved in part to Wisconsin), 1660s, named for their principal settlement, the name of which is from Oneida onenyote', literally "erected stone," containing -neny- "stone" and -ot- "to stand."

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oneiric (adj.)

"of or pertaining to dreams," 1859, from Greek oneiros "a dream" (see oneiro-) + -ic.

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