1861, "motion of a receding wave;" see back (adv.) + wash (v.). As "residue in a glass or bottle of beer after drinking most of it," by 1897.

"wooded or partially uncleared and unsettled districts in remote regions," 1709, North American English; see back (adj.) + wood (n.) in the sense "forested tract." 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]

1847, plural of Modern Latin bacterium, from Greek bakterion "small staff," diminutive of baktron "stick, rod, staff, cudgel." So called because the first ones observed were rod-shaped. Introduced as a scientific word 1838 by German naturalist Christian Gottfried Ehrenberg. A classical plural sometimes also erroneously used as a singular.

The Greek word is from a PIE *bak- "staff used for support, peg" (compare Latin baculum "rod, walking stick;" Irish bacc, Welsh bach "hook, crooked staff;" Middle Dutch pegel "peg, pin, bolt"). De Vaan writes, "Since *b was very rare in PIE, and Celtic shows an unexplained geminate, we are probably dealing with a loanword from an unidentified source."

"virus that parasitizes a bacterium by infecting it and reproducing inside it," 1921, from French bactériophage (1917), from bacterio-, combining form of bacteria, + -phage.