'Pong' is the French interpretation of the 'bang' from a gun, therefore meaning explosive. 'Ass' is a commonly used word referring to being like one's posterior. So therefore, pong ass means:
Use 1) A person with explosive gas.
Use 2) A term for very good sex.
This can also simply be used as a random insult, as in example use 3.
Use 1) He is such a pong ass after eating fried food.
Use 2) Damn, she's some good pong ass.
Use 3) You pong ass.