Etymology
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in re (prep.)

"in the matter of, in the (legal) case of," c. 1600, probably from Duns Scotus; Latin, from re, ablative of res "property, goods; matter, thing, affair," from Proto-Italic *re-, from PIE *reh-i- "wealth, goods" (source also of Sanskrit rayi- "property, goods," Avestan raii-i- "wealth").

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in-gather (v.)

also ingather, 1570s, from in (adv.) + gather (v.). Related: Ingathered; ingathering (1530s).

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lead-in (n.)

1913, in electrical wiring, from verbal phrase; see lead (v.1) + in (adv.). General sense of "introduction, opening" is from 1928, originally in music.

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in-service (adj.)

also inservice, 1928, from in (prep.) + service (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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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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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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