O.E.
twa, fem. and neut. form of
twegen "two" (see
twain), from P.Gmc.
*twai (cf. O.S., O.Fris.
twene, twa, O.N.
tveir, tvau, Du.
twee, O.H.G.
zwene, zwo, Ger.
zwei, Goth.
twai), from PIE
*duwo (cf. Skt.
dvau, Avestan
dva, Gk.
duo, L.
duo, O.Welsh
dou, Lith.
dvi, O.C.S.
duva, first element in Hittite
ta-ugash "two years old").
Twofold is O.E.
tweofeald. Dance style
two-step is recorded from 1900.
Twofer is first recorded 1911 (originally in ref. to cigars), from
two for (a dollar, etc.). Two-faced "deceitful" first recorded 1619.
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; fig. sense of "lacking substance or depth" is attested from 1934.
[too] 
,