Etymology
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in (adj.)
"that is within, internal," 1590s, from in (adv.). 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.
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in (adv., prep.)

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.

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sign-in (n.)
1968, from the verbal phrase; see sign (v.) + in (adv.).
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in-itselfness (n.)
1879, in philosophy; see in (adv.) + itself + -ness.
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trade-in (n.)
1917, in reference to used cars, from verbal phrase, from trade (v.) + in (adv.).
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in-flight (adj.)
also inflight, "during or within a flight," 1945, from in (prep.) + flight.
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plug-in (adj.)

"designed to be plugged into a socket," 1922, from the verbal phrase, from plug (v.) + in (adv.).

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in-depth (adj.)

"profoundly, with careful attention and deep insight," 1967, from the adjective phrase (attested by 1959); see in (adv.) + depth.

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in-patient (n.)
also inpatient, "person lodged and fed, as well as treated, at a hospital or infirmary," 1760, from in (adj.) + patient (n.). As an adjective by 1890.
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