The term was used, and possibly coined, by Deandra "Sweet Dee" Reynolds in season 7, episode 5 (Storm of the Century) It's Always Sunny in Philadelphia.
Deep-Dish-Dickin is when you are a brotha (black man), and you are with a beautiful black woman. During your time together you perform intercourse (makin sweet love), and give her long hard strokes deep inside.
The feelings are enjoyable for both parties, and provide a great time for those involved.