Etymology
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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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in utero 
1713, Latin, literally "in the uterus," from ablative of uterus (see uterus).
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in memoriam 

Latin, literally "in memory of," from accusative of memoria "memory" (from PIE root *(s)mer- (1) "to remember"). The phrase was much-used in Latin writing; Tennyson's poem of that name (published in 1850) seems to have introduced the phrase to English.

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in absentia (adv.)
Latin, literally "in (his/her/their) absence" (see absence). By 1831 in English, earlier in legal Latin.
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in-house (adj.)
also inhouse, 1955, from in (prep.) + house (n.).
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in vitro 
1892, scientific Latin; "in a test tube, culture dish, etc.;" literally "in glass," from Latin vitrum "glass" (see vitreous).
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in-service (adj.)
also inservice, 1928, from in (prep.) + service (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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hang in (v.)
"persist through adversity," 1969, usually with there; see hang (v.) + in (adv.).
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in-going (adj.)
also ingoing, 1825, from in + going. Probably a modern formation unrelated to Middle English in-going (n.) "act of entering" (mid-14c.), from ingo "to go in, enter," from Old English ingan (past tense ineode), equivalent of German eingehen, Dutch ingaan.
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