Etymology
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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-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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in-migration (n.)
1942, American English, in reference to movement within the same country (as distinguished from immigration), from in (prep.) + migration.
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log in (v.)
verbal phrase, 1963 in the computing sense, from log (v.2) + in (adv.).
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