The Collaborative International Dictionary of English v.0.48:

lower-class \lower-class\ adj.
   same as low-class. Contrasted with middle-class and
   upper-class.

   Syn: low-class.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lower-class
    adj 1: occupying the lowest socioeconomic position in a society
           [syn: lower-class, low-class] [ant: middle-class,
           upper-class]