Etymology
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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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twelve-month (n.)
"a year," Old English twelf-monð; see twelve + month.
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twelfth (adj., n.)

"next in order after the eleventh; an ordinal numeral; being one of twelve equal parts into which a whole is regarded as divided;" late 14c., with -th (1), altering Middle English twelfte, from Old English twelfta, from twelf (see twelve). The earlier form is cognate with Old Norse tolfti, Danish tolvte, Old Frisian twelefta, Dutch twaalfde, Old High German zwelifto, German zwölfte .

As a noun meaning "a twelfth part," from 1550s. Twelfth Night is Old English twelftan niht "Twelfth Night," the eve of Epiphany, which comes twelve days after Christmas, formerly an occasion of social rites and a time of merrymaking.

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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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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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-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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*dwo- 

Proto-Indo-European root meaning "two."

It forms all or part of: anadiplosis; balance; barouche; between; betwixt; bezel; bi-; binary; bis-; biscuit; combination; combine; deuce; deuterium; Deuteronomy; di- (1) "two, double, twice;" dia-; dichotomy; digraph; dimity; diode; diphthong; diploid; diploma; diplomacy; diplomat; diplomatic; diplodocus; double; doublet; doubloon; doubt; dozen; dual; dubious; duet; duo; duodecimal; duplex; duplicate; duplicity; dyad; epididymis; hendiadys; pinochle; praseodymium; redoubtable; twain; twelfth; twelve; twenty; twi-; twice; twig; twilight; twill; twin; twine; twist; 'twixt; two; twofold; zwieback.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."

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duodenary (adj.)

"relating to the number twelve, twelve-fold," 1766, from Latin duodenarius "containing twelve," from duodeni "twelve each," from duodecim "twelve" (see dozen).

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duodenum (n.)

"first portion of the small intestine," late 14c., also duodene, from Medieval Latin duodenum digitorium "space of twelve digits," from Latin duodeni "twelve each" (from duodecim "twelve;" see dozen). Coined by Gerard of Cremona (d. 1187) in "Canon Avicennae," a loan-translation of Greek dodekadaktylon, literally "twelve fingers long." The intestine part was so called by Greek physician Herophilus (c. 353-280 B.C.E.) for its length, which is about equal to the breadth of twelve fingers. The classical plural is duodena.

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dodecahedron (n.)

"solid having twelve faces," 1560s, from Greek dōdeka "twelve" (see dodeca-) + hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit." Related: Dodecahedral.

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