ass rest
When a butt is parked on --and hangs over the edge of-- a horizontal object (staff, polearm, railing, pulled-down pantyhose, etc. ...) or a surface in a way where it emphasizes the shape of the rear. Analogue of elbow rest, but when particular attention is drawn to the ass.
When creating a similar effect by holding it or framing it with hands, this becomes instead ass support.