What's some good comebacks if a guy slaps your ass? Funny ones. He's you're friend goofing around?

My friend thinks it's funny to slap my ass at school and it's fine cause he's my friend but I wanna be able to have a funny good comeback when he does it. Any suggestions?
What's some good comebacks if a guy slaps your ass? Funny ones. He's you're friend goofing around?
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