A joke some people play at parties. It consists of poking your finger first into your own mouth and then into your "friend"'s ear. It was originated on the "SpongeBob SquarePants" cartoon show. SpongeBob played it on Patrick Star.
Patrick: "Did you stick your tongue in my ear, SpongeBob?"
SpongeBob: "No, Patrick--I gave you a Wet Willie!"
Patrick: "Oh, okay. I thought you were doing something gross!"