Etymology
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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 toto (adv.)

Latin, "as a whole, wholly, completely, utterly, entirely," from toto, ablative of totus "whole, entire" (see total (adj.)); "always or nearly always with verbs of negative sense" [Fowler].

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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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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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sign-in (n.)

"act or action of signing in," 1968, originally in reference to a shift of work, from the verbal phrase, which is attested by 1930 as "secure admission of by signing a register;" 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-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-flight (adj.)

also inflight, "during or within a flight," 1945, from in (prep.) + flight.

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in situ 

1740, Latin, literally "in its (original) place or position," from ablative of situs "site" (see site (n.)).

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