Moon-willy Share definition Flag 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!" Moon-willy by lardy91 January 19, 2015 0 0 Get merch Get the Moon-willy mug. Buy Now — $32.95 See more merch