WordNet (r) 3.0 (2006):

lower class
    n 1: the social class lowest in the social hierarchy [syn:
         lower class, underclass]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "lower class":
   bourgeoisie, cog, common people, common run, common sort,
   commonage, commonality, commonalty, commoners, commons, creature,
   flunky, follower, hoi polloi, inferior, junior, laborers,
   lightweight, linendrapers, lower classes, lower middle class,
   lower orders, lumpen proletariat, masses, middle class,
   middle orders, ordinary people, pawn, peasantry, plain folks,
   plain people, proletariat, rank and file, second fiddle, secondary,
   shopkeepers, small tradesmen, subaltern, subordinate,
   the lower cut, the other half, the third estate, third stringer,
   toilers, toiling class, underling, understrapper,
   upper middle class, vulgus, working class, working people,
   yes-man