n.

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.

In addition to the idioms beginning with two

• two bits
• two can play at that game
• two cents
• two left feet, have
• two of a kind
• two shakes of a lamb's tail
• two strikes against
• two strings to one's bow
• two ways about it
• two wrongs do not make a right

also see:

• fall between the crack (two stools)
• for two cents
• game that two can play
• goody-two-shoes
• in two shakes
• it takes two
• kill two birds with one stone
• know all the answers (a thing or two)
• lesser of two evils
• like as two peas in a pod
• no two ways about it
• of two minds
• put two and two together
• that makes two of us
• thing or two
• wear two hats