Etymology
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merry widow 

"amorous or designing widow," 1907, from the English title of Franz Lehar's operetta "Die Lustige Witwe" (1905). "The Lusty Widow" would have been more etymological (see lust (n.)), but would have given the wrong impression in English. Meaning "a type of wide-brimmed hat" (popularized in the play) is attested from 1908.

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pis aller (n.)
"last resource, what one would do at the worst," 1670s, French, literally "to go worse," from pis "worse," from Latin peius, neuter of peior "worse" (see pejorative) + aller "to go" (see alley (n.1)).
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fifth column (n.)
1936, from Gen. Emilio Mola's comment at the siege of Madrid during the Spanish Civil War that he would take the city with his four columns of troops outside it and his "fifth column" (quinta columna) in the city.
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civil disobedience (n.)

coined 1866 by Thoreau as the title of his essay originally published (1849) as "Resistance to Civil Government."

If the alternative is to keep all just men in prison, or give up war and slavery, the State will not hesitate which to choose. If a thousand men were not to pay their tax-bills this year, that would not be a violent and bloody measure, as it would be to pay them, and enable the State to commit violence and shed innocent blood. This is, in fact, the definition of a peaceable revolution, if any such is possible. [Thoreau]
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drawing room (n.)

"room appropriated for the reception of company," 1640s, short for withdrawing room (16c.; see withdraw), into which ladies would retire after dinner. Earlier in the sense of "private room" as draw-chamber (mid-15c.); drawyng chaumber (early 15c.).

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modus vivendi (n.)

"mode of living," especially a working agreement between contending parties, 1844, Latin, literally "way of living or getting along" (see modus).

Modus vivendi is any temporary compromise that enables parties to carry on pending settlement of a dispute that would otherwise paralyse their activities. [Fowler]
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two bits (n.)

"quarter dollar," 1730, in reference to the Mexican real, a large coin that was divided into eight bits; see bit (n.1). Compare piece of eight (under piece (n.1)). Two bits thus would have equaled a quarter of the coin. Hence two-bit (adj.) "cheap, tawdry," first recorded 1929.

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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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world war (n.)

attested by 1898 as a speculation.

If through fear of entangling alliances the United States should return the Philippines to Spain, Mr. Page asserted that the predatory nations would swoop down upon them and a world war would result. [New York Times, Dec. 16, 1898]

Applied to the first one almost as soon as it began in 1914 ("England has Thrown Lot with France in World War" — headline, Pittsburgh Press, Aug. 2, 1914). World War I was coined 1939, replacing Great War as the most common name for it; First World War, World War II, and Second World War all also are from 1939. Old English had woruldgewinn, woruldgefeoht, both of which might be translated "world war," but with "world" in the sense of "earthly, secular."

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fast and loose 
described as "a cheating game played with a stick and a belt or string, so arranged that a spectator would think he could make the latter fast by placing a stick through its intricate folds, whereas the operator could detach it at once." [James O. Halliwell, "Dictionary of Archaic and Provincial Words," 1847]. The figurative sense (1550s) is recorded earlier than the literal (1570s).
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