inyoudaoicibaDictYouDict[in 词源字典]
in: [OE] In is a widespread preposition amongst the Indo-European languages. Greek had en, Latin in (whence French and Italian en and Spanish in), and amongst modern languages German and Dutch have in, Swedish i, Welsh yn, and Russian v, all of which point back to an original Indo-European *en or *n. The adverb in was not originally the same word; it comes from a conflation of two Old English adverbs, inn and inne, both ultimately related to the preposition in. (An inn is etymologically a place ‘in’ which people live or stay.)
=> inn[in etymology, in origin, 英语词源]
inyoudaoicibaDictYouDict
Old English in (prep.) "in, into, upon, on, at, among; about, during;" inne (adv.) "within, inside," from Proto-Germanic *in (cognates: Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE *en "in" (cognates: Greek en, Latin in "in, into," Old Irish in, Welsh yn-, Old Church Slavonic 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) 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.