Leper's Tip

Alright listen up, buttercup. This joke is funny because lepers, like you after I'm done with you, are known for losing body parts due to their disease. The leper is telling the prostitute that she can keep the tip, in this case, referring to the tip of his penis, because he doesn't need it anymore, and because prostitutes usually get paid tips for their services. You wouldn't get it, but I hope you do now. Now go away, I need to sleep.