twoyoudaoicibaDictYouDict[two 词源字典]
two: [OE] Two is an ancient word, traceable right back to Indo-European *duwo. Amongst its other descendants were Greek dúo, Latin duo (source of English deuce, double, dual, duet, duo, etc), Russian dva, and Welsh dau. In the Germanic languages, aside from English, it has evolved into German zwei, Dutch twee, Swedish två, and Danish to. Two represents the Old English feminine and neuter forms twā; the masculine twegen has become twain.
=> between, deuce, double, dual, duet, duo, twain, twelve, twenty, twice, twig, twilight, twin, twist[two etymology, two origin, 英语词源]
two (adj.)youdaoicibaDictYouDict
Old English twa "two," fem. and neuter form of twegen "two" (see twain), from Proto-Germanic *twa (cognates: 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, variant of dwo "two" (cognates: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva "two," first element in Hittite ta-ugash "two years old").

Two-fisted is from 1774. 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.