Etymology
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caveat emptor 

1520s, Latin, literally "let the buyer beware;" see caveat and second element of exempt (adj.).

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black swan (n.)

proverbial for "something extremely rare or non-existent" (late 14c.) is from Juvenal ["Sat." vi. 164], but the real thing turned up in Australia (Chenopsis atratus).

"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]

Blue dahlia also was used 19c. for "something rare and unheard of."

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Hobson's choice (n.)
English university slang term, supposedly a reference to Thomas Hobson (c. 1544-1631), Cambridge stable manager who let horses and gave customers a choice of the horse next in line or none at all. Phrase popularized c. 1660 by Milton, who was at Cambridge from 1625-29.
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Marie Antoinette 

(1755-1793), queen consort of Louis XVI; as the name for a decorative style of France in that period, by 1887. She likely did not say "let them eat cake" (see cake (n.)). The city of Marietta, Ohio, U.S., founded in 1788, was named for her in honor of Louis XVI's financial support of the American Revolution.

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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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cold war (n.)

"nonhostile belligerency," used in print October 1945 by George Orwell; popularized in U.S. c. 1947 by U.S. statesman Bernard Baruch (1870-1965). Hence hot war (1947).

More than any other time in history, mankind faces a crossroads. One path leads to despair and utter hopelessness. The other, to total extinction. Let us pray we have the wisdom to choose correctly. [Woody Allen, from "My Speech to the Graduates," 1979]
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civil war (n.)

"battles among fellow citizens or within a community," from civil in a sense of "occurring among fellow citizens" attested from late 14c. in batayle ciuile "civil battle," etc. The exact phrase civil war is attested from late 15c. (the Latin phrase was bella civicus). An Old English word for it was ingewinn. Ancient Greek had polemos epidemios.

Early use typically was in reference to ancient Rome. Later, in England, to the struggle between Parliament and Charles I (1641-1651); in U.S., to the War of Secession (1861-1865), an application often decried as wholly inaccurate but in use (among other names) in the North during the war and boosted by the popular "Battles and Leaders of the Civil War" series published 1884-87 in "Century Magazine."

"The war between the States," which a good many Southerners prefer, is both bookish and inexact. "Civil war" is an utter misnomer. It was used and is still used by courteous people, the same people who are careful to say "Federal" and "Confederate." "War of the rebellion," which begs the very question at issue, has become the official designation of the struggle, but has found no acceptance with the vanquished. To this day no Southerner uses it except by way of quotation .... "The war of secession" is still used a good deal in foreign books, but it has no popular hold. "The war," without any further qualification, served the turn of Thucydides and Aristophanes for the Peloponnesian war. It will serve ours, let it be hoped, for some time to come. [Basil L. Gildersleeve, "The Creed of the Old South," 1915]
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