- in O.E. in (prep.) "in, into, upon, on, at, among; about, during;" inne (adv.) "within, inside," from P.Gmc. *in (Cf. O.Fris, Du., Ger., Goth. in, O.N. i), from PIE *en "in" (Cf. Gk. en, L. in "in, into," O.Ir. in, Welsh yn-, O.C.S. on-). As an adjective from 1590s.The forms merged in Middle English. Modern sense distinction between in and on is from later Middle English. Sense of "holding power" (the in party) first recorded c.1600; that of "exclusive" (the in-crowd, an IN-JOKE (Cf. in-joke)) is from 1907 (in-group); that of "stylish, fashionable" (the in thing) is from 1960. The noun sense of "influence, access" (have an in with) first recorded 1929 in American English. In-and-out "copulation" is attested from 1610s.
Etymology dictionary. 2014.