The space in between the button on your jeans and your stomach. This is a pocket superior to all others as it allows you to be precariously close to your package
without the suspicion of playing with yourself. Child molesters and senile old folks seem to have deeper third pockets than most people, but they could be venturing beyond the realm of the third pocket.
Dustin could not have been more content than right now as he walks down the street with his hand in his third pocket.