A punch bunny is a really hot girl who doesn't have a boyfriend: the reason why is because she is so stupid you can really only do one thing with her, punch it - and leave it. IE - a playboy bunny.
Friend: "Who was that girl you took home the other night, that blonde with the huge tits and great turd cutter?"
Answer:"I have no idea. Sex was great, but then she started talking. Another dumb punch bunny."