Etymology
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piece de resistance (n.)

"most important piece or feature," 1831, from French pièce de résistance, originally "the most substantial dish in a meal." Literally "piece of resistance;" there seems to be disagreement as to the exact signification.

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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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de rigueur 
1849, French, literally "of strictness," thus "according to obligation of convention." See rigor.
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de minimis 
Latin, literally "of little things," thus, "so minor as to not be worth regarding."
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de facto 

Latin, literally "in fact, in reality," thus, "existing, but not necessarily legally ordained or morally right;" from facto, ablative of factum "deed, act" (see de +  fact).

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de novo 

Latin, "anew, afresh," hence "from the beginning," from ablative of novus "new" (see new).

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coup de foudre (n.)

"sudden, unforeseen occurrence," 1779, from French coup de foudre, literally "stroke of lightning," also "love at first sight" (see coup).

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hors de combat (adv.)
1757, French, literally "out of combat." Hors (prep.) "out, beyond," is from Latin foris (adv.) "outside," literally "out of doors" (see foreign). De is from Latin de "of." For combat see combat (n.). A similar expression from French is hors concours "out of competition" (1884), of a work of art in an exhibition.
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van de Graaff 
in reference to an electrostatic charge generator, 1934, named for U.S. physicist R.J. van de Graaff (1901-1967).
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tour de force (n.)
"feat of strength," 1802, French; see tour (n.) + force (n.).
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