Etymology
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gandy dancer 

"railroad maintenance worker," 1918, American English slang, of unknown origin; dancer perhaps from movements required in the work of tamping down ties or pumping a hand-cart, gandy perhaps from the name of a machinery belt company in Baltimore, Maryland.

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hors de combat (adv.)

1757, French, literally "out of combat." Hors (prep.) "out, beyond," is from Latin foris (adv.) "outside," literally "out of doors" (see foreign). De is from Latin de "of." For combat see combat (n.). A similar expression from French is hors concours "out of competition" (1884), of a work of art in an exhibition.

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big bang (n.)

hypothetical explosive beginning of the universe, developed from the work of astronomers Monsignor Georges Henri Joseph Édouard Lemaître and George Gamow; the phrase is first attested 1950 (said to have been used orally in 1949), used by British astronomer Fred Hoyle (1915-2001) in an attempt to explain the idea in laymen's terms.

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skid row (n.)

place where vagabonds, low-lifes, and out-of-work men gather in a town, 1921, with reference to Seattle, Washington, U.S., a variant of skid road "track of skids along which logs are rolled" (1851); see skid (n.); the sense of which was extended to "part of town inhabited by loggers" (1906), then, by hobos, to "disreputable district" (1915); probably shaded by the notion of "go downhill."

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absolute zero (n.)

"lowest possible temperature which the nature of heat admits" (determined to be –273 centigrade, –458 Fahrenheit), the idea dates back to 1702 and its general value was guessed to within a few degrees soon thereafter, but not precisely discovered until Lord Kelvin's work in 1848. It was known by many names, such as infinite cold, absolute cold, natural zero of temperature; the term absolute zero was among them by 1806.

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gung ho (adj.)

also gung-ho, gungho, 1942, slang motto of Carlson's Raiders (2nd Marine Raider Battalion, under Lt. Col. Evans Carlson, 1896-1947), U.S. guerrilla unit operating in the Pacific in World War II, from Chinese kung ho "work together, cooperate." Widely adopted in American English 1959.

Borrowing an idea from China, Carlson frequently has what he calls 'kung-hou' meetings .... Problems are threshed out and orders explained. [New York Times Magazine, Nov. 8, 1942]
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back seat (n.)

also back-seat, 1832, originally of coaches, from back (adj.) + seat (n.). Used figuratively for "less or least prominent position" by 1868. Back-seat driver "passenger who gives the driver unwanted advice" is attested by 1923.

You know him. The one who sits on the back seat and tells the driver what to do. He issues a lot of instructions, gives advice, offers no end of criticism and doesn't do a bit of work. ["The Back Seat Driver," Wisconsin Congregational Church Life, May 1923]
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time zone (n.)

by 1885, from time (n.) + zone (n.). As in Britain and France, the movement to regulate time nationally came from the railroads.

Previous to 1883 the methods of measuring time in the United States were so varied and so numerous as to be ludicrous. There were 50 different standards used in the United States, and on one road between New York and Boston, whose actual difference is 12 minutes, there were three distinct standards of time. Even small towns had two different standards one known as "town" or local time and the other "railroad" time.
... At noon on November 18, 1883, there was a general resetting of watches and clocks all over the United States and Canada, and the four great time zones, one hour apart, into which the country was divided came into being. So smoothly did the plan work that the general readjustment was accomplished without great difficulty and it has worked satisfactorily ever since. [Railroad Trainman, September 1909]
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bleeding heart (n.)

name applied to several types of flowering plant, 1690s; see bleeding (adj.) + heart (n.).

In the American English sense of "person liberally and excessively sympathetic" (especially toward those the speaker or writer deems not to deserve it) is attested by 1936 in the work of popular conservative newspaper columnist Westbrook Pegler (1894-1969), who first used it in reference to his own feelings about the Republican Party but by 1938 regularly deployed it against the Roosevelt administration and also as a modifier (bleeding-heart liberal) in his "Fair Enough" column: 

And I question the humanitarianism of any professional or semi-pro bleeding heart who clamors that not a single person must be allowed to hunger, but would stall the entire legislative program in a fight to jam through a law intended, at the most optimistic figure, to save 14 lives a year. ["Fair Enough," in Freemont (Ohio) Messenger, Jan. 8, 1938]

Bleeding in a figurative sense of "generous" is attested from late 16c., and the notion of one's heart bleeding as a figure of emotional anguish is from late 14c.; the exact image here may be the "bleeding heart of Jesus."

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son of a bitch 

1707 as a direct phrase, but implied much earlier, and Old Norse had bikkju-sonr. Abbreviated form SOB from 1918; form sumbitch attested in writing by 1969.

Abide þou þef malicious!
Biche-sone þou drawest amis
þou schalt abigge it ywis!
["Of Arthour & of Merlin," c. 1330]

"Probably the most common American vulgarity from about the middle of the eighteenth century to the middle of the twentieth" [Rawson].

Our maid-of-all-work in that department [indecency] is son-of-a-bitch, which seems as pale and ineffectual to a Slav or a Latin as fudge does to us. There is simply no lift in it, no shock, no sis-boom-ah. The dumbest policeman in Palermo thinks of a dozen better ones between breakfast and the noon whistle. [H.L. Mencken, "The American Language," 4th ed., 1936, p.317-8]

Elsewhere, complaining of the tepidity of the American vocabulary of profanity, Mencken writes that the toned-down form son-of-a-gun "is so lacking in punch that the Italians among us have borrowed it as a satirical name for an American: la sanemagogna is what they call him, and by it they indicate their contempt for his backwardness in the art that is one of their great glories."

It was in 1934 also that the New York Daily News, with commendable frankness, in reporting a hearing in Washington at which Senator Huey P. Long featured, forsook the old-time dashes and abbreviations and printed the complete epithet "son of a bitch." [Stanley Walker, "City Editor," 1934]
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