A Willy Moon is when someone flashes their ass at you and their dick flops out between their legs at the same time
"Dave became enraged when his Grandad gave him a willy moon"
by Blaise Noctem March 16, 2015
Get the Willy Moon mug.
A willy that only comes out at night. A willy that doesn't often have sex during the day.
Morning time:
"Oh Chris, hop on!"

"Ahhh nah babe I'm playing on gta!"

"Oh you're such a moon-willy!"
by lardy91 January 19, 2015
Get the Moon-willy mug.