"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]