shut-in n.

Middle English shitten, sheten, "close (a door, window, gate, etc.); lock, fasten closed," from Old English scyttan "to put (a bolt) in place so as to fasten a door or gate, bolt, shut to; discharge, pay off," from West Germanic *skutjan (source also of Old Frisian schetta, Middle Dutch schutten "to shut, shut up, obstruct"), from PIE root *skeud- "to shoot, chase, throw." Related: Shutting.

The meaning "to close by folding or bringing together" is from mid-14c. That of "prevent ingress and egress" is from mid-14c. The sense of "to set (someone) free (from)," by c. 1500, is obsolete except in dialectal phrases such as get (or be) shut of (attested by 1570s). To shut (one's) mouth "desist from speaking" is recorded from mid-14c.

As a past-participle adjective, "made fast, closed, enclosed," by late 15c. As a noun, "action, time, or place of shutting," by 1660s.

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.