Slapping the peen is referred to as "Masturbating" by thugs and others affiliated with thugs. This combination comes together to produce a good party anecdote, for example "Do I got a story for you guys... So the other night I was slapping the ole' peen, and right when the wife walks in I "YOU!" all up on her face.
Aye homey, once I got home, I pulled my pants down and started 'Slapping the peen'