Etymology
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capital letter (n.)

late 14c.; see capital (adj.). So called because it is at the "head" of a sentence or word.

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Petri dish (n.)

1892, named for German bacteriologist Julius Petri (1852-1922), who first devised it c. 1887.

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boob tube (n.)

"television set," U.S. slang, 1959, from boob "stupid person" + slang tube (n.) "television, television programming;" the original sets had vacuum tubes in them. It seems to have been popularized, if not coined, by William Ewald, television columnist for the UPI news wire, in a column resigning his position, which was widely reprinted in US newspapers in August, who headlined it variously.

To all those who say TV provides the sort of fare that they, ordinary people, like, I say shame on you for reveling in your ordinariness.
To all those who say there is room for all kinds of tastes, nonsense again. There is obviously a hierarchy of values in life—without it, we become vegetables. If you prefer to squander your free time on Lawrence Welk, The Texan, The Price Is Right and other drivel, it may be time for you to question your values. ['Sorehead' Calls It Quits As Boob Tube Chronicler," Tucson Daily Sun, Aug. 14, 1959]
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king's evil (n.)

"scrofula," late 14c.; it translates Medieval Latin regius morbus. The name came about because the kings of England and France claimed and were reputed to be able to cure it by their touch. In England, the custom dates from Edward the Confessor and was continued through the Stuarts (Charles II touched 90,798 sufferers) but was ended by the Hanoverians (1714).

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Bright's disease 

"chronic nephritis," 1831, so called for English physician Richard Bright (1789-1858), who in 1827 first described it.

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plain clothes (n.)

"ordinary dress of civil life" (as opposed to military uniform), 1822; in reference to police detectives, it is attested from 1842. Also plainclothes.

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Dead Sea 

lake of the River Jordan, mid-13c., from dead (adj.); its water is 26 percent salt (as opposed to 3 or 4 percent in most oceans) and supports practically no life. In the Bible it was the "Salt Sea" (Hebrew yam hammelah), also "Sea of the Plain" and "East Sea." In Arabic it is al-bahr al-mayyit "Dead Sea." The ancient Greeks knew it as he Thalassa asphaltites "the Asphaltite Sea." Latin Mare Mortum, Greek he nekra thalassa (both "The Dead Sea") referred to the sea at the northern boundaries of Europe, the Arctic Ocean.

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Ruy Lopez (n.)

type of chess opening, 1876, from Ruy López de Segura (fl. 1560), Spanish bishop and writer on chess, who developed it.

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cui bono 

a Latin phrase from Cicero. It means "to whom for a benefit," or "who profits by it?" not "to what good purpose? for what use or end?" as is sometimes said. From cui "to? for whom?," an old form preserved here in the dative form of the interrogative pronoun quis "who?" (from PIE root *kwo-, stem of relative and interrogative pronouns) + bono "good" (see bene-).

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guinea pig (n.)

rodent native to South America, 1660s. It does not come from Guinea and has nothing to do with the pig. Perhaps so called either because it was brought back to Britain aboard Guinea-men, ships that plied the triangle trade between England, Guinea, and South America [Barnhart, Klein], or from its resemblance to the young of the Guinea-hog "river pig" [OED], or from confusion of Guinea with the South American region of Guyana (but OED is against this). Pig probably for its grunting noises. In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in medical experiments (by 1865).

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