sit-in n.

"To be or remain in that posture in which the weight of the body rests upon the posteriors" [OED], Middle English sitten, from Old English sittan "occupy a seat, be seated, sit down, seat oneself; remain, continue; settle, encamp; lie in wait; besiege" (class V strong verb; past tense sæt, past participle seten), from Proto-Germanic *setjan (source also of Old Saxon sittian, Old Norse sitja, Danish sidde, Old Frisian sitta, Middle Dutch sitten, Dutch zitten, Old High German sizzan, German sitzen, Gothic sitan), from PIE root *sed- (1) "to sit."

With past tense sat (formerly also set, which is now restricted to dialect, and sate, now archaic); and past participle sat, formerly sitten. The meaning "to have a seat in a legislative assembly" is from late 14c.; in reference to the assembly, "to hold a session," from 1510s. The sense of "pose" for a portrait, etc., is by 1530s.

As short for babysit (v.) by 1966. The specific sense of "occupy a judicial seat" (Old English) is the notion in sit in judgement. The meaning "have a certain position or direction" is from c. 1200; of winds, "to blow from" (a certain quarter), 1590s, from the notion of "to be in."

To sit back "be inactive" is from 1943. To sit on one's hands originally was "withhold applause" (1926), later generalized to "do nothing" (by 1959). To sit around "be idle, do nothing" is by 1915, American English. To sit out "not take part, make oneself an exception" is from 1650s.

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.

the adverb in attached to a verb as a word-forming element, by 1960, abstracted from sit-in, which is attested from 1941 in reference to protests and 1937 in reference to labor union actions (which probably was influenced by sit-down strike) but was popularized in reference to civil disobedience protests aimed at segregated lunch counters.

As a word-forming element it was extended first of other types of protests, then by 1965 to any sort of communal gathering (such as love-in, attested by 1967; slim-in, for dieters, 1973). In labor actions it was perhaps less useful: "a mass of workers calling in sick to absent themselves in protest" was called both a sick-out (1970) and a sick-in (1974).