Etymology
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backflip (n.)

"a backwards somersault in the air," 1906; see back (adj.) + flip (n.).

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backdrop (n.)

1883 in theatrical argot, "painted cloth hung at the back of a stage as part of the scenery," from back (adj.) + drop (n.).

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backbone (n.)
"spine, vertebral column," early 14c., from back (n.) + bone (n.). Figurative sense of "firmness of purpose, strength of character" is by 1843.
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backless (adj.)
"without a back," 1919, in reference to women's gowns and dresses, earlier of benches, from back (n.) + -less.
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backpedal (v.)
also back-pedal, 1883, in bicycling; see back (adv.) + pedal (v.). Related: Backpedalling (1887).
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backstairs (n.)
"stairs at the back of a structure," 1650s, from back (adj.) + stairs (see stair). Figurative use is attested earlier (1640s).
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backorder 
also back-order, by 1980 (n.); 1985 (v.); see back (adj.) + order. Related: Backordered.
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backing (n.)
1590s, "support at the back;" 1640s, "retreat;" verbal noun from back (v.). Physical sense of "anything placed at or attached to the back of something else" is from 1793. Meaning "musical accompaniment" is recorded from 1937.
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hardback (n.)
"type of book bound in stiff boards," 1954, from hard (adj.) + back (n.).
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sway-backed (adj.)
1670s, according to OED of Scandinavian origin, perhaps related to obsolete Danish sveibaget. See sway (v.) + back (n.).
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