Short for lumpenproletariat. A catch-all term used primarily by self-styled Marxists to describe people "below" the proletariat. The lumpen include all people who might belong to the working class, if they worked. The unemployed, disabled, prisoners, and homeless are all part of the lumpen.
Also, the name of a political and cultural zine from Chicago.
The Black Panther Party attempted to organize the lumpen. Eldridge Cleaver: "Armed with this ideological perspective and method, Huey transformed the Black lumpenproletariat from the forgotten people at the bottom of society into the vanguard of the proletariat."