Emily Dickinson Lexicon

CRUP'PER, n. [Fr. croupiere; It. groppiera; Sp. grupera; from croupe, groppa, grupa, a ridge, the buttocks of a horse. See Croup.]

1. In the manege, the buttocks of a horse; the rump.
2. A strap of leather which is buckled to a saddle, and passing under a horse's tail, prevents the saddle from being cast forward on to the horse's neck.