The Collaborative International Dictionary of English v.0.48:

Crupper \Crup"per\ (kr?p"p?r in U.S.; kr?p"?r in Eng.), n. [F.
   croupi?re, fr. croupe. See Croup the rump of a horse.]
   [Written also crouper.]
   1. The buttocks or rump of a horse.
      [1913 Webster]

   2. A leather loop, passing under a horse's tail, and buckled
      to the saddle to keep it from slipping forwards.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Crupper \Crup"per\, v. t.
   To fit with a crupper; to place a crupper upon; as, to
   crupper a horse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crupper
    n 1: a strap from the back of a saddle passing under the horse's
         tail; prevents saddle from slipping forward