"Lumpen" is a shortening of the German noun "Lumpenproletariat" (the lowest section of the proletariat), which is a combination of the German "Lumpen" (rags or ragamuffins), and French "proletariat" (lowest socioeconomic class). Marxist theory popularized "lumpenproletariat" in English.
"Lumpen" became a separate word in English by the 1940s, and it usually refers to
Sometimes these last two definitions are meant as slurs; other times it becomes one of those "reclaimed" words that people who are members of a group can use about themselves without insult being meant. For example, a number of the Black Panthers in the late 1960s formed a singing group which they called the Lumpen, in the hopes of spreading their political message through soul music. Michael Torrence, one of the members, recalls that their name was specifically chosen to evoke the '"brothers on the block," the disenfranchised, angry underclass in the ghetto.' However, such coinages as "lumpenbourgeoisie," "lumpen-intelligentsia" and "lumpen politariat," all found online while researching this word, show that it is not always limited to a socioeconomic underclass.
Lumpen (lumpen.com) is an independent magazine headquartered in Chicago, Illinois. The name seems to have been chosen for its associations with revolutionary movements and people who "refuse to have their lives determined by the hegemonic forces of monoculture," as magazine founder Ed Marszewski put it.
On a different note, British dictionary Cambridge Advanced Learner's Dictionary also gives an unrelated definition of the word "lumpen" as "lumpy and heavy." I can't find any dictionaries from any other English-speaking area that use this meaning. And Webster 1913's missing-from-E2 entry also notes that a lumpen can be "(Zool.) The European eelpout; - called also lumper."