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

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "groundling":
   Adamite, Babbitt, Philistine, arriviste, attender, audience,
   auditory, being, body, boor, bounder, bourgeois, cad, cat, chap,
   character, churl, claqueur, clown, congregation, creature,
   customer, deadhead, duck, earthling, epicier, fellow, gallery,
   guttersnipe, guy, hand, head, hired applauder, homo, hooligan,
   house, human, human being, ill-bred fellow, individual, joker,
   life, living soul, looby, lout, low fellow, man, mortal, moviegoer,
   mucker, nose, nouveau riche, one, orchestra, party, parvenu,
   pass holder, peasant, person, personage, personality, pit,
   playgoer, ribald, rough, roughneck, rowdy, ruffian, single,
   somebody, someone, soul, spectator, standee, tellurian, terran,
   theater, theatergoer, upstart, vulgarian, vulgarist, worldling,
   yokel