"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.

"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."

"next in order after the nineteenth; an ordinal numeral; being one of twenty equal parts into which a whole is regarded as divided;" 16c., from twenty + -th (1), replacing Middle English twentithe, from Old English twentigoða. The Twentieth Century Limited was an express train from New York to Chicago 1902-1967.

"1 more than nineteen, twice ten; the number which is one more than nineteen; a symbol representing this number;" Old English twentig "group of twenty," from twegen "two" (from PIE root *dwo- "two") + -tig "group of ten" (see -ty (1)). Cognate with Old Saxon twentig , Old Frisian twintich, Dutch twintig, Old High German zweinzug, German zwanzig. Gothic twai tigjus is even more transparent: literally "two tens."

The card game twenty-one (1790) is from French vingt-et-un (1781). Twenty-twenty hindsight is first recorded 1962, a figurative use of the Snellen fraction for normal visual acuity, expressed in feet. The guessing game of twenty questions is recorded from 1786 (a late 19c. parlor variation on it was called clumps).

of unknown origin; OED and Barnhart give earliest date as 1925, but the "Dictionary of American Slang" gives a first reference of 1874 (but without citation and I can't find it), which, if correct, would rule out the usual theory that it is from the proper name of T.W. Earp, a student at Oxford c. 1911, who kindled wrath "in the hearts of the rugger-playing stalwarts at Oxford, when he was president of the Union, by being the last, most charming, and wittiest of the 'decadents.' " [Rawson]

"Mean to say you never heard of Sinzy? Why, he's one of the greatest characters in this town. He's a terrible twerp to look at — got a face like bad news from home, but I guess he's the best jazz piano player in the world." [Julian Street, "Cross-Sections," 1923]