IPA: /ˈkrʌpər/
KK: /ˈkrʌpər/
The back part of a horse's body, located just above the tail.
The rider adjusted the saddle to fit comfortably on the horse's crupper.
Crupper originates from Old French *croupier*, meaning the hindquarters of a horse. It refers specifically to the part of a saddle that goes over the horse's hindquarters, thus connecting the term to its original meaning.
Think of the Old French word for the hindquarters of a horse, which helps you remember that a crupper is a part of the saddle that fits over that area.