Etymology
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greenback (n.)
"U.S. dollar bill," 1862, so called from the time of their introduction, from green (adj.) + back (n.); bank paper money printed in green ink had been called this since 1778 (as opposed to redbacks, etc.).
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feedback (n.)
1920, in the electronics sense, "the return of a fraction of an output signal to the input of an earlier stage," from verbal phrase, from feed (v.) + back (adv.). Transferred use, "information about the results of a process" is attested by 1955.
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backlog (n.)
also back-log, 1680s, "large log placed at the back of a fire" to keep the blaze going and concentrate the heat; see back (adj.) + log (n.1). Figurative sense of "something stored up for later use" is first attested 1883, but this and the meaning "arrears of unfulfilled orders" (1932) might be from, or suggested by, log (n.2).
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flashback (n.)
also flash-back, 1903 in reference to fires in engines or furnaces, from verbal phrase (1902), from flash (v.) + back (adv.). Movie plot device sense is from 1916. The hallucinogenic drug sense is attested in psychological literature from 1970, which means probably hippies were using it a few years before.
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backhand (adj.)
1690s, "having the hand turned backward;" see back (adv.) + hand (n.). By 1894 in reference to handwriting that flows at a back-slant. As a verb, by 1857. As a noun, in reference to tennis, 1890, short for backhand stroke or volley. The figurative adjectival sense of "indirect" is from c. 1800. Related: Backhanded; backhanding.
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backbiting (n.)
c. 1200, bacbitunge, "the sin of secretly attacking one's character or reputation through envy," from back (adj. or n.) + biting. Related: back-bite (v.) early 14c.; back-biter (c. 1200). The notion is of injury in a manner comparable to biting from behind. As an adjective Old English had bæcslitol; another old word for it was back-wounding (c. 1600).
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backfire (n.)
1832, American English, originally "a fire deliberately lit ahead of an advancing wildfire to deprive it of fuel," from back (adj.) + fire (n.). As a verb in this sense, recorded from 1886. The noun meaning "premature ignition in an internal-combustion engine" is first recorded 1897. AS a verb, of schemes, plans, etc., "to affect the initiator rather than the intended object" it is attested from 1912, a figurative use from the accidental back-firing of firearms.
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setback (n.)

also set-back, 1670s, "reversal, check to progress," from the verbal phrase, attested mid-15c. as "withhold;" see set (v.) + back (adv.). Backset (1721) is used in the same sense. The meaning "space between a building and a property line or roadway" is from 1916. To set (someone) back "cost" (a certain sum of money) is from 1900.

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backwoods (n.)

"wooded or partially uncleared and unsettled districts in remote regions," 1709, North American English; see back (adj.) + wood (n.) in the sense "forested tract." As an adjective, from 1784.

BACKWOODSMEN. ... This word is commonly used as a term of reproach (and that, only in a familiar style,) to designate those people, who, being at a distance from the sea and entirely agricultural, are considered as either hostile or indifferent to the interests of the commercial states. [John Pickering, "A Vocabulary, or Collection of Words and Phrases Which Have Been Supposed to be Peculiar to the United States of America," Boston, 1816]
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aback (adv.)

c. 1200, "toward the rear," a contraction of Old English on bæc "backward, behind, at or on the back;" see see a- (1) + back (n.). Now surviving mainly in taken aback, which originally was a nautical expression in reference to a vessel's square sails when a sudden change of wind flattens them back against the masts and stops the forward motion (1754). The figurative sense from this, "suddenly or unexpectedly checked or disappointed," is by 1792.

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