The Collaborative International Dictionary of English v.0.48:

Groundling \Ground"ling\, n. [Ground + -ling.]
   1. (Zool.) A fish that keeps at the bottom of the water, as
      the loach.
      [1913 Webster]

   2. A spectator in the pit of a theater, which formerly was on
      the ground, and without floor or benches.
      [1913 Webster]

            No comic buffoon to make the groundlings laugh.
                                                  --Coleridge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

groundling
    n 1: in Elizabethan theater: a playgoer in the cheap standing
         section