proletariat n.

1650s (n.) "member of the lowest or poorest of a community;" 1660s (adj.) "of or belonging to the lowest of people," hence "mean, vile, vulgar;" with -ian + Latin proletarius "citizen of the lowest class" (as an adjective, "relating to offspring"), from proles "offspring, progeny" (see prolific). In ancient Rome, according to the traditional division of the state, the proletarius was one of the propertyless people, exempted from taxes and military service, who served the state only by having children. The modern political sense of proletarian is by 1851.

1897, from German Lumpenproletariat, coined by Marx, who used it in the sense of "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.

Marx used it first, apparently, in 1850 in German newspaper articles collected and republished in 1895 as "Die Klassenkämpfe in Frankreich 1848-1850." Its secondary sense of "boorish, stupid people" led to lumpen- being taken as a word-forming element meaning "unenlightened."