c. 1600 (intransitive) "to return;" 1808 (transitive) "to recover (something);" from get (v.) + back.(adv.). Meaning "retaliate" is attested by 1888.

also fatback, cut of pork, 1903, from fat + back (n.). So called because taken from the back of the animal.

also back-seat, 1832, originally of coaches, from back (adj.) + seat (n.). Used figuratively for "less or least prominent position" by 1868. Back-seat driver "passenger who gives the driver unwanted advice" is attested by 1923.

You know him. The one who sits on the back seat and tells the driver what to do. He issues a lot of instructions, gives advice, offers no end of criticism and doesn't do a bit of work. ["The Back Seat Driver," Wisconsin Congregational Church Life, May 1923]

also backache, "dull or continuous pain in the back," c. 1600, from back (n.) + ache (n.).

in reference to a return to simpler ways of living, without modern electricity, manufacturing, conveniences, etc., 1915, from the adverbial phrase; see nature (n.).

also canvasback, 1785 as a type of North American duck, so called for the color of the back. Earlier as an adjective for a type of garment made of expensive stuff in front and cheap canvas in the back (c. 1600); from canvas (n.) + back (n.).

1898 as a type of chair, from ladder (n.) + back (n.).

"words pronounced or written backwards or nearly so," 1860, from back (adj. or adv.) + slang (n.).

also, especially as an adjective, backdoor, "door at the rear of a building or other structure," 1520s, from back (adj.) + door (n.).

As an adjective, "devious, shady, illegal," by 1640s. The notion is of business done out of public view. The association with sodomy is from at least 19c.; compare also back-door man "a married woman's lover," African-American vernacular, early 20c.

in figurative sense of "withdraw a charge," 1859, American English, from the notion of descending a ladder, etc. (such a literal sense is attested by 1849); from back (v.) + down (adv.).