Etymology
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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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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 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 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 situ 
1740, Latin, literally "in its (original) place or position," from ablative of situs "site" (see site (n.)).
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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 toto (adv.)
Latin, "as a whole, wholly, completely, utterly, entirely," from toto, ablative of totus "whole, entire" (see total (adj.)); "always or nearly always with verbs of negative sense" [Fowler].
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in utero 
1713, Latin, literally "in the uterus," from ablative of uterus (see uterus).
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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 vivo 
1898, Latin; "within a living organism," from vivere "to live" (see vital).
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