Etymology
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old hat (adj.)

"out of date," 1911, from old + hat. As a noun phrase, however, it had different sense previously. The "Classical Dictionary of the Vulgar Tongue" (1796) defines it as, "a woman's privities, because frequently felt."

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hard hat (n.)
also hardhat, hard-hat, late 14c., "helmet," from hard (adj.) + hat (n.). From 1935 as "derby hat;" meaning "safety helmet" is from 1953; used figuratively for "construction worker" from 1970.
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high hat (n.)
1839, "tall hat;" also used synechdochically for men who wear such hats; figurative meaning "swelled head" is from 1923. Drum set sense is from 1934.
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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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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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polka-dot 

"pattern consisting of dots of uniform size and arrangement," especially on fabric, 1851 (polka-spot and polka-dotted are used in 1849), when they were in fashion, from polka (n.) + dot (n.). Named for the dance, for no reason except its popularity, which led to many contemporary products and fashions taking the name (polka hat, polka-jacket, etc.). They had a revival in fashion c. 1873. Related: Polka-dots.

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Dobermann pinscher (n.)

breed of domestic dog, originally used in police work, 1911 (as pincher Dobermann from 1907), named for Ludwig Dobermann, 19c. German dog-breeder in Thuringia. Pinscher "fox terrier" seems to be a 19c. borrowing from English pinch (see Kluge).

Der Kutscher aus gutem Hause verschafft sich, wie er kann und wenn er kann, einen ganz kleinen englischen Pinscher, der den Pferden sehr gut gut folgt und die großen Dänen von ehedem ersetzt hat, aus J.J. Rousseau's Zeit, der von dem dänischen Hunde umgerannt wurde, wie ihr wißt. ["Paris, oder, Das Buch der Hundert und Ein," Volume 6, Theodor Hell (pseud.), Potsdam, 1833]
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