Etymology
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Gordian knot (n.)
1560s, tied by Gordius (Greek Gordios), first king of Phrygia in Asia Minor and father of Midas, who predicted the one to loosen it would rule Asia. Instead, Alexander the Great cut the Gordian knot with his sword; hence the extended sense (1570s in English) "solve a difficult problem in a quick, dramatic way."
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mother-of-pearl (n.)

"nacreous inner layer of the shell of various bivalve mollusks," c. 1500, translating Medieval Latin mater perlarum, with the first element perhaps connected in popular imagination with obsolete mother (n.2) "dregs." Compare Italian madreperla, French mère-perle, Dutch parelmoer, German Perlmutter, Danish perlemor. It is the stuff of pearls but in a layer instead of a mass.

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id est 
Latin, literally "that is (to say)," from id "that," neuter of is, from PIE pronominal stem *i- (see yon). For est, see is. Usually abbreviated i.e. "to write, or even to say, this in the full instead of in the abbreviated form is now so unusual as to convict one of affectation" [Fowler]. It introduces another way to say something already said, not an example of it (which is e.g.).
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violon d'Ingres (n.)

"an occasional pastime, an activity other than that for which one is well-known, or at which one excells," 1963, from French, literally "Ingres' violin," from the story that the great painter preferred to play his violin (badly) for visitors instead of showing them his pictures.

Une légende, assez suspecte, prétend que le peintre Ingres état plus fier de son jeu sur le violon, jeu qui était fort ordinaire, que de sa peinture, qui l'avait rendu illustre. [Larousse du XXe Siecle, 1931]
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color-blindness (n.)

also colour-blindness, "incapacity for perceiving certain colors due to an absence or weakness of the sensation upon which the power of distinguishing them depends," 1844, the native word, used in England instead of French daltonisme (by 1828), after English chemist John Dalton (1766-1844), who published a description of it in 1794. From color (n.) + blindness.

The continental philosophers have named it Daltonism, a name which has been strongly objected to by almost every English writer who has discussed the subject, on the ground of the inexpediency and undesirableness of immortalizing the imperfections or personal peculiarities of celebrated men by title of this kind. ... The name "Color-Blindness," proposed by Sir D. Brewster, seems in every respect unexceptionable. [Littell's Living Age, vol. v, April 1845]

Noted as inexact (very few people who can see are blind to all color), "the term is applied with much laxity to any constitutional inability to discriminate between colours" [OED]. In figurative use, with reference to race or ethnicity, it is attested from 1866, American English. Related: color-blind (adj.), which is attested from 1854.

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