When a man or women goes up to another person (most likely someone they know) and takes there two fingers and flips someones breast or "titty" and say "nip flip"
Warning: don't do this to a stranger, because the results in the joke my very from funny to violent.
Bob went up to his best friend jimmy and flipped up jimmy's nipple and said nip flip. Jimmy confused and laughing said what the hell are you doing. Bob reply's "i just gave you a nip flip".