Crupper

(krōōp′per in U. S.; krŭp′per in Eng.), n. [Fr. croupière, from croupe: Sp. grupéra, from grupa; It. groppiera, from groppa. See CROUP, hind-quarters.]

1. The buttocks or rump of a horse.
2. A strap of leather which is buckled to a saddle, and, passing under a horse’s tail, prevents the saddle from slipping or being thrown forward on the horse’s neck. [Written also crouper.]