Definitions by The guy on the couch.

musty slap 

When a man lathers one of his hands on his testicles, giving them a warm and sour texture, and in a light, playful manner, slaps another person.

John was being a little bitch at the game, so afterwards I came up behind him and gave him the old Musty slap

musty slap by The guy on the couch. June 8, 2009