A loop or strap on a horse's saddle that passes under the horse's tail to prevent the saddle from sliding forward.
From Old French croupe meaning 'rump' (from Frankish kruppja). The -er suffix may indicate an agent noun (something that goes around the croup/rump), evolving in English equestrian vocabulary.
A crupper is a simple engineering solution that's been used for centuries—it exploits the fact that you can't slide a loop backward under a horse's tail, which is why saddles stay put even on steep terrain!
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