"plant not valued for use or beauty," Old English weod, uueod "grass, herb, weed," from Proto-Germanic *weud- (source also of Old Saxon wiod, East Frisian wiud), of unknown origin. Also applied to trees that grow abundantly. Meaning "tobacco" is from c. 1600; that of "marijuana" is from 1920s. The chemical weed-killer is attested by 1885.
"to clear the ground of weeds," late Old English weodian "to weed," from the source of weed (n.). Figurative use by c. 1400. Related: Weeded; weeding; weeder.