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."
The term was invented from a character in the Anime "Shin Chan", a mother who holds a stuffed bunny against the wall and punches it repeatedly in the gut to relieve her numerous frustrations.
The term is used to describe someone you know must be going home at night and punching some stuffed animal against the wall in secrecy.
"Shes little tightly strung, a real bunny puncher"
"I can't believe Gail remains so eerily pleasant, how does she do it?"
"I bet shes a bunny puncher!"