Noun. A term used to describe the act of two men having double penetration sex with a woman, whereas each man has at least a 12 inch penis, and are standing. With each thrust, the men's testicles are repeatedly hitting each other, making a thumping sound. Commonly causes Sticky Bumper Balls.
Guy1: "Your mother likes bumper balls, doesn't she?"
Guy2: "How'd you know?"
Guy1: "Nick and I were not at your house to watch the dog."