twentyyoudaoicibaDictYouDict[twenty 词源字典]
twenty: [OE] Twenty is etymologically ‘two tens’. The second syllable, -ty, is related to ten, and twen- goes back to the same source as two, although its precise ancestry is not clear.
=> ten, two[twenty etymology, twenty origin, 英语词源]
twenty (n.)youdaoicibaDictYouDict
Old English twentig "group of twenty," from twegen "two" (see 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.