-in (1)

a Middle English merger of Old English in (prep.) "in, into, upon, on, at, among; about, during;" and Old English inne (adv.) "within, inside," from Proto-Germanic *in (source also of Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE root *en "in." The simpler form took on both senses in Middle English.

Sense distinction between in and on is from later Middle English, and nuances in use of in and at still distinguish British and American English (in school/at school). Sometimes in Middle English shortened to i.

The noun sense of "influence, access (to power or authorities)," as in have an in with, is first recorded 1929 in American English. to be in for it "certain to meet with something unpleasant" is from 1690s. To be in with "on friendly terms with" is from 1670s. Ins and outs "intricacies, complications of an action or course" is from 1660s. In-and-out (n.) "copulation" is attested from 1610s.

"act of sitting in," 1937, from the verbal phrase, "take part, have a place" as a player in a game (1590s); see sit (v.) + in (adv.). The verbal phrase is attested by 1936 in reference to session musicians, "join in" with a band or orchestra.

As "occupy a building," it is attested by 1937 in reference to union action (sit-in strike is attested from 1938), by 1941 in reference to student protests. To sit in is attested from 1868 in the sense of "attend, be present;" sit-in as a noun meaning "a large crowd" (in a restaurant or taproom) is by 1869. To sit in is from 1919 specifically as "attend as an observer."