Similar to a Wet Willy except you are sticking a penis in someone's ear, not a wet finger.
Guy: Man, Frank schooled Bob by giving him a Wet Johnny!
Guy2: Is Frank gay?
when you are in a bathroom stall, take a large amount of toilet paper or paper towels (anything 'dunkable'), dunk it in the pissy/shitty water, and throw at target.
Guy 1: timmy threw some wet paper towels at me while i was taking a dump, so i gave him a wet johnny right in the face!
Guy 2: it's the only proper revenge for being hit with a wet paper towel while pooping.