Etymology
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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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levy (n.2)
1829, colloquial shortening of elevenpence (see eleven). In U.S. before c. 1860, a Spanish real or an equivalent amount of some other money (about 12 and a half cents).
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eleventh (adj., n.)

"next in order after the tenth; an ordinal numeral; being one of eleven equal parts into which a whole is regarded as divided;" late 14c., eleventhe, superseding earlier ellefte (c. 1300), enlefte (early 13c.), from Old English endleofta; see eleven + -th (1). Eleventh hour "last moment, just before it is too late" is in Old English, from the parable of the laborers in the vineyard (Matthew xx.1-16); as an adjective by 1829.

But aboute the elleventhe hour he wente out and founde other stondynge, and he seide to hem, what stonden ye idel heere al dai? [Wyclif, Matthew xx] 
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twelve (adj., n.)

"1 more than eleven, twice six; the number which is one more than eleven; a symbol representing this number;" Old English twelf "twelve," literally "two left" (over ten), from Proto-Germanic *twa-lif-, a compound of *twa- (from PIE root *dwo- "two") + *lif- (from PIE root *leikw- "to leave"). Compare eleven. Cognate with Old Saxon twelif, Old Norse tolf, Old Frisian twelef, Middle Dutch twalef, Dutch twaalf, Old High German zwelif, German zwölf, Gothic twalif. Outside Germanic, an analogous formation is Lithuanian dvylika, with second element -lika "left over."

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*leikw- 
Proto-Indo-European root meaning "to leave."

It forms all or part of: delinquent; derelict; eclipse; eleven; ellipse; ellipsis; elliptic; lipo- (2) "lacking;" lipogram; loan; paralipsis; relic; relict; reliction; relinquish; reliquiae; twelve.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit reknas "inheritance, wealth," rinakti "leaves;" Greek leipein "to leave, be lacking;" Latin linquere "to leave;" Gothic leihvan, Old English lænan "to lend;" Old High German lihan "to borrow;" Old Norse lan "loan."
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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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-ty (1)
suffix representing "ten" in cardinal numbers that are multiples of 10 (sixty, seventy, etc.), from Old English -tig, from a Germanic root (cognates: Old Saxon, Dutch -tig, Old Frisian -tich, Old Norse -tigr, Old High German -zug, German -zig) that existed as a distinct word in Gothic (tigjus) and Old Norse (tigir) meaning "tens, decades." Compare tithe (n.).

English, like many other Germanic languages, retains traces of a base-12 number system. The most obvious instance is eleven and twelve which ought to be the first two numbers of the "teens" series. Their Old English forms, enleofan and twel(eo)f(an), are more transparent: "leave one" and "leave two."

Old English also had hund endleofantig for "110" and hund twelftig for "120." One hundred was hund teantig. The -tig formation ran through 12 cycles, and could have bequeathed us numbers *eleventy ("110") and *twelfty ("120") had it endured, but already during the Anglo-Saxon period it was being obscured.

Old Norse used hundrað for "120" and þusend for "1,200." Tvauhundrað was "240" and þriuhundrað was "360." Older Germanic legal texts distinguished a "common hundred" (100) from a "great hundred" (120). This duodecimal system is "perhaps due to contact with Babylonia" [Lass, "Old English"].
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hendeca- 
word-forming element meaning "eleven," from Latinized form of Greek hendeka "eleven," from hen, neuter of heis "one," from PIE *hems-, from root *sem- (1) "one; as one, together with" + deka "ten" (from PIE root *dekm- "ten").
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undecennial (adj.)
"occurring every 11 years," 1858, in reference to solar activity cycle, from Latin undecim "eleven" + ending from biennial, etc.
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possession (n.)

mid-14c., possessioun, "act or fact of holding, occupying, or owning; a taking possession, occupation," also "thing possessed, that which is possessed, material or landed property" (in plural, goods, lands, or rights owned), from Old French possession "fact of having and holding; what is possessed;" also "demonic possession," and directly from Latin possessionem (nominative possessio) "a seizing, possession," noun of action from past-participle stem of possidere "to possess" (see possess).

The legal property sense is earliest; the demonic sense in English, "state of being under the control of evil spirits or of madness," first is recorded 1580s. Phrase possession is nine (or eleven) points of the law is out of a supposed 10 (or 12). With eleven from 1640s; with nine from 1690s.

St. Jerome in his 'Life of St. Hilarion' has given us a graphic account of the courage with which that saint confronted, and the success with which he relieved, a possessed camel. [Lecky, "History of European Morals"]
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