early 15c., "to curl up in a ball, to gather into a lump" (implied in lumped), from lump (n.). Transitive meaning "to put together in one mass or group" is from 1620s. Related: Lumped; lumping (from 1705 as a slang present-participle adjective meaning "great, big"):

LUMPING. Great. A lumping pennyworth; a great quantity for the money, a bargain. He has got a lumping pennyworth; frequently said of a man who marries a fat woman. [Grose, "Classical Dictionary of the Vulgar Tongue," 3rd edition, 1796]

1897, from German Lumpenproletariat (1850), from Marx, who coined it and used it to mean "the rabble, poorest of the working class," "who make no contribution to the workers' cause" [OED]. From German lump "ragamuffin," which is related to lumpen "a rag, tatter," probably ultimately related to English lump (n.). With proletariat. Its secondary sense of "boorish, stupid people" led to lumpen- being taken as a word-forming element meaning "unenlightened."