The Collaborative International Dictionary of English v.0.48:

Lumpenproletariat \Lump`en*pro*le*tar"i*at\, n. (Politics)
   the lowest level of the proletariat, comprising unskilled
   workers, the unemployed, and the dispossessed, alienated from
   the class with with they would normally identify and having
   little or no class solidarity; -- an important element in
   Marxist theory.
   [PJC]

WordNet (r) 3.0 (2006):

lumpenproletariat
    n 1: (Marxism) the unorganized lower levels of the proletariat
         who are not interested in revolutionary advancement