The Collaborative International Dictionary of English v.0.48:

low-class \low-class\ adj.
   1. Occupying the lowest socioeconomic position in a society.
      Contrasted with middle-class and upper-class.
      [Narrower terms: propertyless, wage-earning,
      working-class, blue-collar] Also See: lowborn,
      proletarian, propertyless.

   Syn: lower-class (vs. upper-class).
        [WordNet 1.5]

   2. characteristic of the lower classes. [Narrower terms:
      non-U, vulgar] PJC]

WordNet (r) 3.0 (2006):

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