Crotch rope
From wipipedia.org

In Japanese bondage a crotch rope (Sukaranbo) is a rope harness tied around the hips and groin in such a way as to rub against the genitals. The aim is to generate thoughts of sexual submission and excitement in the person thus tied.
Crotch ropes are also common in Western-style bondage. They appear regularly in the drawings of Robert Bishop.