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.

To flip a female or male’s titties

To flash your nipples aggressively at someone out of spite or anger, as opposed to flipping them off.