n. The buttocks of a horse; the rump.n. A. strap of leather which is buckled at one end to the back of a saddle, or to the saddle of a harness, and at the other passes by a loop under the horse's tail, to prevent the saddle from slipping forward. Also crouper. See cut under harness.To put a crupper on: as, to crupper a horse.n. Nautical, the train-tackle bolt in a gun-carriage.