crupper UK /ˈkɹʌpə/US /ˈkɹʌpɚ/noun 3verb 1forms 3 noun3 •A strap, looped under a horse's tail, used to stop a saddle from slipping.Our knight did bear no less a pack / Of his own buttocks on his back: / Which now had almost got the upper- / Hand of his head, for want of crupper. — 1662 •The buttocks or rump, especially of a horse. •A piece of armour covering the hindquarters/buttocks of a horse. verb1 •To fit with a crupper; to place a crupper upon. formscruppered · cruppering · cruppers