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Old English twa, fem. and neuter form of twegen "two" (see twain), from Proto-Germanic *twai (cf. Old Saxon and Old Frisian twene, twa, Old Norse tveir, tvau, Dutch twee, Old High German zwene, zwo, German zwei, Gothic twai), from PIE *duwo (cf. Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, first element in Hittite ta-ugash "two years old").
Dance style two-step is recorded from 1900. Twofer is first recorded 1911 (originally in reference to cigars), from two for (a dollar, etc.). Two cheers for _____, expressing qualified enthusiasm first recorded 1951 in E.M. Forster's title "Two Cheers for Democracy." Two-dimensional is recorded from 1883; figurative sense of "lacking substance or depth" is attested from 1934.
Attractively feminine; sexy: Most female doctors aren't as, uh, young. No, ah, say twisty might be more like it (1970s+)