Etymology
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horseback (n.)
"the back of a horse," especially the part upon which a rider sits, late 14c., from horse (n.) + back (n.). The alternative formerly was described in jest as footback [Century Dictionary].
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blowback (n.)
also blow-back, 1883, in reference to flames in enclosed spaces (firearms, furnaces, etc.), from blow (v.1) + back (adv.). Sense in reference to convert actions, etc., is from 1978.
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backspace (adj.)

also back-space, 1899, in reference to keyboarding, from back (adv.) + space.

We have had the pleasure of examining one of the 1899 model Hammond typewriters, with the new back-space key. This new feature is certainly an improvement in the machine. [The Phonetic Journal, March 11, 1899]
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stickleback (n.)
c. 1400, from Old English sticel "prick, sting, goad, thorn" (from Proto-Germanic *stik- "pierce, prick, be sharp;" see stick (v.)) + back (n.).
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razorback (n.)

also razor-back, type of hog with a sharp ridge-like back, 1849, from razor (n.) + back (n.). Especially of feral hogs in the U.S. South. Also used of narrow ridges of land.

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runback (n.)

also run-back, by 1944 in U.S. football, "a run to advance the ball after catching a kick or punt," from the verbal phrase; see run (v.) + back (adv.).

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comeback (n.)

also come-back, 1889 as "verbal retort," from the verbal phrase; see come + back (adv.). Meaning "recovery, return to former position or condition after retirement or loss" is attested from 1908, American English.

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saddleback (n.)

1540s and thereafter in various senses in reference to things (landforms, oysters, etc.) shaped like a saddle, from saddle (n.) + back (n.). Related: Saddle-backed.

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backdoor (adj.)
also back-door, "devious, shady, illegal," 1640s. The notion is of business done out of public view. The noun back door in the literal sense is from 1520s, from back (adj.) + door. The association with sodomy is from at least 19c.; compare also back-door man "a married woman's lover," African-American vernacular, early 20c.
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fallback 
also fall-back, as a noun, "a reserve," 1851, from verbal phrase, from fall (v.) + back (adv.), which is attested in the sense of "retreat" from c. 1600. As an adjective, from 1767 as a type of chair; 1930 as "that may be used in an emergency."
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