When a girl jerks her torso forward in a strong and sudden motion when they are in a stationary position. This is usually done by a girl when she is dancing -or- when she is riding the guy during sexual intercourse.
Such a motion (a whop) if strong enough will usually make the girl's breasts and buttcheeks jiggle if the girl has prominent enough of these features.
My girlfriend is on top of me riding me and whopping repeatedly.