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).