Etymology
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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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back-breaking (adj.)
"physically demanding" (of manual labor), 1849; see back (n.) + break (v.).
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backdate (v.)
also back-date, "assign a date to earlier than the actual one," by 1881 (implied in back-dated), from back (adv.) + date (v.1). Compare antedate. Related: Backdated; backdating.
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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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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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backer (n.)
"supporter, one who aids and abets," 1580s, agent noun from back (v.).
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backfill (n.)
1900, "material taken from an excavation used to fill a depression," 1900, from back fill (v.), which is attested by 1880; see back (adv.) + fill (v.).
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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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backflip (n.)

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

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back-formation (n.)

also back formation, "word formed from an existing word, often by removal of a suffix or supposed suffix," by 1887, from back (adv.) + formation.

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