Online Etymology Dictionary

back up (v.): 1767, "stand behind and support," from back (v.) + up. The noun meaning "a standby, a reserve" is recorded from 1952 (often written as one word, backup); specific reference to computing is from 1965.
back-ache (n.): c. 1600, from back (n.) + ache (n.).
back-formation (n.): also back formation, by 1887, from back (adv.) + formation.
back-stitch (n.): 1610s, from back (adj.) + stitch (n.).
back-to-nature (adj.): first attested 1915.
backbencher (n.): 1874 in the House of Commons sense, from back (adj.) + bench (n.); occupants of the rear seats being the least-prominent politicians.
backbiting (n.): c. 1200, bacbitunge, from back (adj. or n.) + biting. Related: back-bite (v.), early 14c.; back-biter (c. 1200).
backbone (n.): "spine," early 14c., from back (n.) + bone (n.). Figurative sense of "strength of character" is attested from 1843.
backdate (v.): also back-date, by 1881 (implied in back-dated), from back (adv.) + date (v.1). Compare antedate. Related: Backdated; backdating.
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 at least from 19c.; compare also back-door man "a married woman's lover," African-American vernacular, early 20c.
backdrop (n.): 1913, in U.S. theatrical argot, from back (adj.) + drop (n.).
backer (n.): "supporter," 1580s, agent noun from back (v.).
backfill (n.): 1901 (as backfilling), from back (adj.) + fill (n.). The verb was in use by 1930.
backfire (n.): 1839, American English, originally "a fire deliberately lit ahead of an advancing prairie fire to deprive it of fuel," from back (adj.) + fire (n.). As a verb in this sense, recorded from 1886. The meaning "premature ignition in an internal-combustion engine" is first recorded 1897. 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.
backgammon (n.): 1640s, baggammon, the second element from Middle English gamen, ancestor of game; the first element apparently because pieces sometimes are forced to go "back." Known 13c.-17c. as tables.
background (n.): 1670s, from back (adj.) + ground (n.); original sense was theatrical, later applied to painting. Figurative sense is first attested 1854.
backhand (n.): as a tennis stroke, 1650s, from back (adv.) + hand. As a verb, by 1935. The figurative adjectival sense of "indirect" is from c. 1800. Related: Backhanded; backhanding.
backhoe (n.): by 1928, from back (n. or adj.) + hoe (n.).
backing (n.): 1590s, "support;" 1640s, "retreat;" verbal noun from back (v.). Physical sense of "anything forming a backing to something else" is from 1793. Meaning "musical accompaniment" is recorded from 1940.
backlash (n.): 1815, of machinery, from back (adj.) + lash (n.). In metaphoric sense, it is attested from 1955.
backless (adj.): 1926, in reference to women's clothing, from back (n.) + -less.
backlist (n.): 1946, in publishing, from back (adj.) + list (n.1). As a verb, "to put on the back list," from 1983. Related: Backlisted.
backlog (n.): 1680s, "large log placed at the back of a fire," from 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).
backorder: also back-order, by 1980 (n.); 1985 (v.), from back (adj.) + order. Related: Backordered.
backpack: 1914 as a noun, 1916 as a verb, from back (n.) + pack (n.). Related: Backpacked; backpacking.
backside (n.): c. 1400, from back (adj.) + side (n.). In the specific sense of "rump, buttocks" it is first recorded c. 1500.
backslash (n.): 1982, new punctuation symbol introduced for computer purposes, from back (adj.) + slash (n.).
backslide (v.): in the religious sense, 1580s, from back (adj.) + slide (v.). Related: Backslider; backsliding (1550s).
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]
backstabber (n.): also back-stabber, in the figurative sense, 1839, from back (n.) + agent noun from stab (v.). The verb backstab in the figurative sense is from 1925.
backstage: also back-stage, 1898, from back (adj.) + stage (n.).
backstairs: "stairs at the back of a structure," 1650s, from back (adj.) + stairs (see stair). Figurative use is attested earlier (1640s).
backstop (n.): 1819, in cricket, from back (adj.) + stop (n.). In U.S. baseball, from 1889, "fence behind the catcher;" figurative extension to "catcher on a baseball team" is from 1890. The verb is attested from 1956 in the sense of "support." Related: Backstopped; backstopping.
backstory (n.): c. 1990, from background story.
backstreet (n.): mid-15c., from back (adj.) + street.
backstroke (n.): 1670s, "counter-punch," from back (adj.) + stroke (n.). From 1876 in swimming, from back (n.).
backtalk (n.): also back-talk, "impertinent retort," 1858, originally often used in literary attempts at low Irish idiom, from back (adj.) + talk (n.).
backtrack (v.): "retrace one's steps," figuratively, by 1896, from literal sense, with reference to hunted foxes, from back (adv.) + track (v.). Related: Backtracked; backtracking.
backup: see back up.
backward (adv.): c. 1300, from abakward, from Old English on bæc (see back (adv.)) + -weard adjectival and adverbial suffix (see -ward). Old English had the adverb bæcling. As an adjective, from 1550s. Meaning "behindhand with regard to progress" is first attested 1690s. To ring bells backward (from lowest to highest), c. 1500, was a signal of alarm for fire or invasion, or to express dismay. Another Middle English word for "backward, wrongly" was arseward (c. 1400); Old English had earsling.
backwardness (n.): 1580s, from backward + -ness.
backwards (adv.): 1510s, from backward with adverbial genitive. Figurative phrase bend over backwards is recorded from 1901.
backwash (n.): 1876, "motion of a receeding wave," from back (adj.) + wash (n.).
backwater (n.): late 14c., "water behind a dam," from back (adj.) + water (n.1). Hence flat water without a current near a flowing river, as in a mill race (1820); figurative use of this for any flat, dull place is from 1899.
backwood (n.): 1709, American English, from back (adj.) + wood (n.) "forested tract." Also backwoods. 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]
backyard (n.): also back-yard, 1650s (perhaps early 15c.), from back (adj.) + yard (n.1).
bacon (n.): early 14c., "meat from the back and sides of a pig" (originally either fresh or cured, but especially cured), from Old French bacon, from Proto-Germanic *bakkon "back meat" (source also of Old High German bahho, Old Dutch baken "bacon"). Slang phrase bring home the bacon first recorded 1908; bacon formerly being the staple meat of the working class.
bacteria (n.): 1847, plural of Modern Latin bacterium, from Greek bakterion "small staff," diminutive of baktron "stick, rod," from PIE *bak- "staff used for support" (also source of Latin baculum "rod, walking stick"). So called because the first ones observed were rod-shaped. Introduced as a scientific word 1838 by German naturalist Christian Gottfried Ehrenberg (1795-1876).
bacterial (adj.): 1869, from bacteria + -al (1).
bacteriology (n.): 1884, from German; see bacteria + -ology. Related: Bacteriological (1886). Bacteriological warfare is from 1924.