noun
crupper
A strap on a horse's harness that passes under the tail to prevent the saddle from sliding forward. A specialized equestrian term.
馬に鞍を置くときは、しりがいを忘れずに付けてください。
When putting a saddle on a horse, don't forget to attach the crupper.
Compound of 尻 (shiri, 'buttocks') + がい (kai, 'rope, cord'), referring to the strap that goes under the horse's tail. The kanji 鞦 is a specialized character for this item.