Etymology
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thank you 
polite formula used in acknowledging a favor, c. 1400, short for I thank you (see thank). As a noun, from 1792.
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tu quoque 
Latin, literally "thou also" (or, in modern vernacular, "so are you!"); an argument which consists in retorting accusations.
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etaoin shrdlu 
1931, journalism slang, the sequence of characters you get if you sweep your finger down the two left-hand columns of Linotype keys, which is what typesetters did when they bungled a line and had to start it over. It was a signal to cut out the sentence, but sometimes it slipped past harried compositors and ended up in print.
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hat trick (n.)

in the sports sense, 1879, originally in cricket, "taking three wickets on three consecutive deliveries;" extended to other sports c. 1909, especially ice hockey ("In an earlier contest we had handed Army a 6-2 defeat at West Point as Billy Sloane performed hockey's spectacular 'hat trick' by scoring three goals" ["Princeton Alumni Weekly," Feb. 10, 1941]). So called allegedly because it entitled the bowler to receive a hat from his club commemorating the feat (or entitled him to pass the hat for a cash collection), but the term probably has been influenced by the image of a conjurer pulling objects from his hat (an act attested by 1876). The term was used earlier for a different sort of magic trick:

Place a glass of liquor on the table, put a hat over it, and say, "I will engage to drink every drop of that liquor, and yet I'll not touch the hat." You then get under the table; and after giving three knocks, you make a noise with your mouth, as if you were swallowing the liquor. Then, getting from under the table, say "Now, gentlemen, be pleased to look." Some one, eager to see if you have drunk the liquor, will raise the hat; when you instantly take the glass and swallow the contents, saying, "Gentlemen I have fulfilled my promise: you are all witnesses that I did not touch the hat." ["Wit and Wisdom," London, 1860]
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qui vive 

1726, in on the qui vive "on the alert," from French être sur le qui vive "be on the alert," from the phrase qui voulez-vous qui vive? sentinel's challenge, "whom do you wish to live?" In other words "(long) live who?" meaning "whose side are you on?" (The answer might be Vive la France, Vive le roi, etc.). From qui (from Latin qui "who") + vive, third person singular present subjunctive of vivre, from Latin vivere "to live" (see viva).

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morituri te salutant 
Latin, literally "those about to die salute you," words addressed to emperor by gladiators upon entering the arena. Third person singular is moriturus te salutat, first person singular is moriturus te saluto.
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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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witch hunt (n.)

1853 in the literal sense (witch-hunting is from 1630s), from witch (n.) + hunt (n.). The extended sense is attested from 1919, American English, later re-popularized in reaction to Cold War anti-Communism.

Senator [Lee S.] Overman. What do you mean by witch hunt?
Mr. [Raymond] Robins. I mean this, Senator. You are familiar with the old witch-hunt attitude, that when people get frightened at things and see bogies, then they get out witch proclamations, and mob action and all kinds of hysteria takes place. ["Bolshevik Propaganda," U.S. Senate subcommittee hearings, 1919]
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loose cannon (n.)

in the figurative sense "wildly irresponsible person, potent person or thing freed from usual restraint," by 1896; in the literal sense an object of dread on old warships; the figurative use probably arose from a celebrated scene in a popular late novel by Victor Hugo:

You can reason with a bull dog, astonish a bull, fascinate a boa, frighten a tiger, soften a lion; no resource with such a monster as a loose cannon. You cannot kill it, it is dead; and at the same time it lives. It lives with a sinister life which comes from the infinite. It is moved by the ship, which is moved by the sea, which is moved by the wind. This exterminator is a plaything. [Victor Hugo, "Ninety Three," 1874]
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ish kabibble 

slang phrase meaning, more or less, "I don't care, I don't worry," 1913, of unknown origin, but perhaps derived from Yiddish nisht gefidlt. Said to have been popularized by comedienne Fanny Brice (1891-1951), but earliest references do not mention her.

Chicken pox doesn't poison the wellsprings of one's existence like 'Ish kabibble,' and 'I should worry.!' Do you think it's any fun to bring up children to speak decent English, and then have their conversation strewed with phrases like that and with ain'ts? Do you think I like to hear Robert talking about his little friends as 'de guys' and 'de ginks?' [Mary Heaton Vorse, "Their Little Friends," in Woman's Home Companion, February 1916]
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