Etymology
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de rigueur 
1849, French, literally "of strictness," thus "according to obligation of convention." See rigor.
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de jure 

Latin, literally "of law," thus "legitimate, lawful, by right of law, according to law." Jure is ablative of ius "law" (see de +  just (adj.)).

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pro rata 

"in proportion," from Medieval Latin pro rata (parte) "according to the calculated (share)," from pro "for, in accordance with" (see pro-) + rata, ablative singular of ratus, past participle of reri "to count, reckon" (see rate (n.)).

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kit and caboodle (n.)
also kaboodle, 1870, earlier kit and boodle (1855), kit and cargo (1848), according to OED from kit (n.1) in dismissive sense "number of things viewed as a whole" (1785) + boodle "lot, collection," perhaps from Dutch boedel "property." Century Dictionary compares the whole kit, of persons, "every one" (1785).
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second nature (n.)
late 14c., from Latin secundum naturam "according to nature" (Augustine, Macrobius, etc.), literally "following nature;" from medieval Aristotelian philosophy, contrasted to phenomena that were super naturam ("above nature," such as God's grace), extra naturam ("outside nature"), supra naturam ("beyond nature," such as miracles), contra naturam "against nature," etc.
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invita Minerva 
Latin adverbial phrase, used with reference to literary or artistic creation, "without inspiration," literally "Minerva unwilling;" i.e. "without inspiration from the goddess of wisdom;" ablative fem. of invitus "against the will, unwilling, reluctant," according to de Vaan from PIE compound *n-uih-to- "not turned to, not pursuing," related to the source of invitation. With Minervā, ablative absolute of Minerva.
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a la 
from French à la, literally "to the," hence "in the manner of, according to," from à, from Latin ad "to" (see ad-) + la, fem. of definite article le "the," from Latin ille (fem. illa; see le). Attested in English in French terms from fashion or cookery since late 16c.; since c. 1800 used in native formations with English words or names.
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Davy Jones 

"the spirit of the sea," 1751, first mentioned in Smollett's "The Adventures of Peregrin Pickle" as ("according to the mythology of sailors") an ominous and terrifying fiend who "presides over all the evil spirits of the deep, and is often seen in various shapes, perching among the rigging on the eve of hurricanes, shipwrecks and other disasters." Davy Jones's Locker "bottom of the sea," is 1803, from nautical slang, of unknown origin; second element may be from biblical Jonah, regarded as unlucky by sailors.

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comme il faut 

"according to etiquette," 1756, French, literally "as it should be." From comme "as, like, how," from Old French com, from Vulgar Latin *quomo, from Latin quomodo "how? in what way?," pronominal adverb of manner, related to quam "how much?," qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns). With il, from Latin ille "this" (see le) + faut, third person singular present indicative active of falloir "be necessary," literally "be wanting or lacking" (see fail (v.)).

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a cappella 

1868, earlier alla capella (1824), from Italian, "in the style of Church music, in the manner of the chapel," literally "according to the chapel," from cappella "chapel" (see chapel). Originally in reference to older church music (pre-1600) which was written for unaccompanied voices; applied 20c. to unaccompanied vocal music generally. Italian a is from Latin ad "to, toward; for; according to" (see ad-); alla is a la "to the." Sometimes in the Latin form a capella.

Also denoting "that instruments are to play in unison with the voices, or that one part is to be played by a number instruments." ["Chambers's' Encyclopaedia," 1868]

You are not the first person puzzled by the expression "A Capella," or, at any rate, unable to understand it should signify the exact reverse of what it literally does signify. The chorales in oratorios were invariably accompanied, either by double-bass or the whole band. Hence they were, with perfect correctness, said to be performed "a capella." But, as other chorales, sung as part of the church service, were written in the same and simple style the expression "a capella" came in time to be applied to them also, despite their being sung without any instrumental accompaniment whatever. [The Music World, Sept. 11, 1875]
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