Etymology
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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 vitro 
1892, scientific Latin; "in a test tube, culture dish, etc.;" literally "in glass," from Latin vitrum "glass" (see vitreous).
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hang in (v.)
"persist through adversity," 1969, usually with there; see hang (v.) + in (adv.).
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in forma pauperis 
legal Latin, literally "in the form of a poor person" (thus exempt from certain court fees, etc.), 1590s; see form (n.) + pauper (n.).
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in custodia legis (adv.)
legal Latin, "in the custody of the law," from ablative of custodia "a guarding, watching, keeping" (see custody) + legis, genitive of lex "law" (see legal (adj.)).
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in totidem verbis 
Latin phrase, "in just so many words," that is, "in these very words," from demonstrative of Latin totus "whole, entire" (see total (adj.)) + ablative plural of verbum "word" (see verb).
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in medias res 

Latin, literally "in the midst of things," from medias, accusative fem. plural of medius "middle" (see medial (adj.)) + accusative plural of res "a thing" (see re). From Horace, in reference to narrative technique:

Semper ad eventum festinat, et in medias res,
Non secus ac notas auditorem rapit (etc.)
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in loco parentis 
legal Latin, 1640s in English, literally "in the place of a parent," from loco, ablative of locus "a place" (see locus (n.)) + parentis, genitive of parens "parent" (see parent (n.)).
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