A pocket is where the bottom of a girls ass folds over the top of her thigh whenever she takes a step. You usually have to have a decent ass to have this, and because of that a good way to rate female ass is by checking if they have a pocket.
dude, dude, dude its like totally when someone tries to masturbait through their pocket without anyone noticing
(but someone will notice when the dude has a cream of wheat)