Crotch strap
From wipipedia.org
A crotch strap is a strap that goes across the groin and between the buttocks.
It may be intended to rub against the genitals to cause sexual arousal. It can also be used to ensure that a butt plug or dildo cannot be ejected. On a male, the crotch strap may be split at the front to avoid the genitals, or it may pass over them so as to squash them.
Crotch straps are often fitted to straitjackets to make them more escape-proof.
