Standing in front of your significant other, or someone else, with a big smile, swinging your junk back and forth so it slaps against your legs. Works best right after you get out of the shower.
When Brandon got out of the shower he showed Cassie slappy slappy so she would wake up.
The act of placing one's penis on the unconscious face of another man (i.e. when passed out or asleep)
A very effective and useful taunt
Hey Kuzz, slappy slappy!
I slappy slappied you good last night
Nice passing out...SLAPPY SLAPPY