Word Origin & History
O.E. in "in," inne "within," from P.Gmc. *in (cf. O.Fris, Du., Ger., Goth. in, O.N. i), from PIE *en-/*n (cf. Gk. en, L. in, O.Ir. in, Welsh yn-, O.C.S. on-). Sense of "holding power" (the in party) first recorded 1605; that of "exclusive" (the in-crowd, an in-joke) is from 1907; that of "stylish, fashionable"
(the in thing) is from 1960. The noun sense of "influence, access" (have an in with) first recorded 1929 in Amer.Eng. In-and-out "copulation" is attested from 1610s.