Etymology
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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-between (n.)
1815, "an interval;" also "a person who intervenes," noun use of prepositional phrase, from in (adv.) + between. Related: In-betweener (1912); in-betweenity (1927).
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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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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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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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buy-in (n.)
"act of obtaining an interest in," 1970, from verbal phrase buy in "to purchase a commission or stock" (1826), from buy (v.) + in (adv.). To buy into "obtain an interest in by purchase" (as of stock shares) is recorded from 1680s.
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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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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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drive-in (adj.)

in reference to of restaurants, banks, etc., built to be patronized without leaving one's car, 1929, from the verbal phrase; see drive (v.) + in (adv.). Of movie theaters by 1933 (the year the first one opened, in Camden, New Jersey).

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in-fighting (n.)
1816, in pugilism, the practice of getting at close quarters with an opponent; see in + fighting. Old English infiht (n.) meant "brawl within a house or between members of a household." Middle English had infight (v.) "to attack" (c. 1300); the modern verb infight "fight at close quarters" (1916) appears to be a back-formation from in-fighting. Related: In-fighter (1812).
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