A pocket to pocket come from the description of a woman's pubic hair, which when left unshaven for a long period of time, will stretch from one pocket in her pants to the other; in other words, a GIANT f'ing bush.
Will: "Hey Matt, you hear about your mom's pocket to pocket?"
Matt: "What are you talking about?"
Will: "Your mom has the biggest bush ever, it stretches from pocket to pocket"
Matt: "Fuck you"