"being behind, away from the front, in a backward direction," Middle English, from back (n.) and back (adv.); often difficult to distinguish from these when the word is used in combinations. Formerly with comparative backer (c. 1400), also backermore. To be on the back burner in the figurative sense is from 1960, from the image of a cook keeping a pot there to simmer while at work on another concoction at the front of the stove.
c. 1300, las "a blow, a stroke," later "flexible part of a whip" (late 14c.), possibly imitative; compare lash (v.1), which might be the immediate source of this. Century Dictionary says Irish lasg "a lash, whip, whipping" is of English origin. The lash "punishment by flogging" is from 1690s.
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Harper, D. (n.d.). Etymology of backlash. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/backlash