Etymology
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in-transit (adj.)
1907, from commercial verbal phrase in transit "on the way or passage, while passing from one to another" (1819, earlier in Latin form in transitu), from in + transit (n.).
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be-in (n.)
"a public gathering of hippies" [OED], 1967, from be + in (adv.).
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live-in (adj.)
"residing on the premises," 1950, from live (v.) + in (adv.). To live out was formerly "be away from home in domestic service."
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walk-in (adj.)
1928, "without appointment," from the verbal phrase, from walk (v.) + in (adv.). As a noun, meaning "walk-in closet," by 1946.
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fill-in (n.)
"substitute," 1918 (as an adjective, 1916), from verbal phrase; see fill (v.), in (adv.). Earlier as a noun was fill-up (1811).
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in-country (n.)
"interior regions" of a land, 1560s, from in (prep.) + country.
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lived-in (adj.)
"inhabited, occupied" (sometimes with suggestion of "shabby, disorderly"), 1873, from verbal phrase; see live (v.) + in (adv.).
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stick-in-the-mud (n.)
1852, from verbal phrase, stick (v.) on notion of "one who sticks in the mud," hence "one who is content to remain in an abject condition." The phrase appears in 1730, in city of London court records, as the alias of an accused named John Baker, who with two other men received a death sentence at the Old Bailey in December 1733 for "breaking open the House of Mr. Thomas Rayner, a Silversmith, and stealing thence Plate to a great Value."
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tongue-in-cheek (adv.)

1856, from phrase to speak with one's tongue in one's cheek "to speak insincerely" (1748), suggestive of sly irony or humorous insincerity, perhaps a stage trick to convey irony to the audience.

Hem! Pray, Sir, said he to the Bard, after thrusting his Tongue into a Corner of his Cheek, and rolling his Eyes at Miss Willis, (Tricks which he had caught by endeavouring to take off a celebrated Comedian) were these fine Tragedies of yours ever acted? [anonymous, "Emily, or the History of a Natural Daughter," 1761]
This arietta, however, she no sooner began to perform, than he and the justice fell asleep ; but the moment she ceased playing, the knight waked snorting, and exclaimed,—'O cara! what d'ye think, gentlemen? Will you talk any more of your Pargolesi and your Corelli ?'—At the same time, he thrust his tongue in one cheek, and leered with one eye at the doctor and me, who sat on his left hand—He concluded the pantomime with a loud laugh, which he could command at all times extempore. [Smollett, "The Expedition of Humphrey Clinker," 1771]
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