Swinging Bulldog: The exclucive male act of {mooning} or showing your naked bottom, while fully bent over exposing and shaking your nutts from side to side. Inable to encounter the full effect of a true swinging bulldog you must be able to see your targets behind you between your legs.
Metephor for having sex with a female that is very "loose". The extra vaginal space can be compared to swinging a cat by it's tail in a large room, such as a gymnasium.
Jim: "Yo, I heard you've been banging Samantha. She good in bed?"
Joe: "Dude, that chick's been around. Banging her is like swinging a cat in a gymnasium."