bump:  The earliest recorded sense of bump is ‘swelling, lump’, but the evidence suggests that the primary meaning is ‘knock’, and that this led on to ‘swelling’ as the result of being hit. It is not clear where the word came from, although it may be of Scandinavian origin; no doubt ultimately it imitates the sound of somebody being hit. The verbal sense ‘swell’, now obsolete, is probably responsible for bumper, which originally meant ‘full glass or cup’, and in the 19th century was extended to anything large or abundant (as in ‘bumper crop’).
1560s, "to bulge out;" 1610s, "to strike heavily," perhaps from Scandinavian, probably echoic, original sense was "hitting" then of "swelling from being hit." Also has a long association with obsolete bum "to make a booming noise," which perhaps influenced surviving senses such as bumper crop, for something full to the brim (see bumper). To bump into "meet" is from 1880s; to bump off "kill" is 1908 in underworld slang. Related: Bumped; bumping. Bumpsy (adj.) was old slang for "drunk" (1610s).