This is when a male stands naked with his legs spread and thrusts in a forward and backwards motion making his spam dagger slap against his bare skin. This is often used as a dance of celebration and joy.
Girl with lip piercing: Do you guys want a 3 some?
Craig: Hell yeahhh *performs willy dance*
to unzip trousers and hang knob out and proceed to slap willy up and down with hands in the air shouting yay yay
"paul proceeded to unzip and flop outside the pub and willy dance down the street"