Etymology
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sign-in (n.)
1968, from the verbal phrase; see sign (v.) + in (adv.).
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in fieri 
legal Latin, "in the process of being done," from fieri "to come into being, become," used as passive of facere "to make, do" (from PIE root *dhe- "to set, put").
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in-law (n.)

1894, "anyone of a relationship not natural," abstracted from father-in-law, etc.

The position of the 'in-laws' (a happy phrase which is attributed ... to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. [Blackwood's Magazine, 1894]

The earliest recorded use of the formation is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited. Thus the word originally had a more narrow application; its general extension to more distant relatives of one's spouse is, according to OED "recent colloquial or journalistic phraseology." Middle English inlaue (13c.) meant "one within or restored to the protection and benefit of the law" (opposite of an outlaw), from a verb inlauen, from Old English inlagian "reverse sentence of outlawry."

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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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run-in (n.)

"quarrel, confrontation," 1905, from the verbal phrase; see run (v.) + in (adv.). From 1857 as "an act of running in," along with the verbal phrase run in "pay a short, passing visit." Earlier to run in meant "to rush in" in attacking (1815).

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sit-in 
1936, in reference to session musicians; 1937, in reference to union action; 1941, in reference to student protests. From the verbal phrase; see sit (v.) + in (adv.). To sit in is attested from 1868 in the sense "attend, be present;" from 1919 specifically as "attend as an observer."
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stand-in (n.)
"one who substitutes for another," 1928, from the verbal phrase, attested from 1904 in show business slang in the sense "to substitute, to fill the place of another," from stand (v.) + in (adv.).
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in vivo 
1898, Latin; "within a living organism," from vivere "to live" (see vital).
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in extremis 
"at the point of death," 16c., Latin, literally "in the farthest reaches," from ablative plural of extremus "extreme" (see extreme (adj.)).
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