The 375ml bottle of liquor available for purchase. It is small enough to fit in your back pocket, aka your ass pocket.
Guy: I'm halfway through my ass pocket of Jack daniels Girl: What is an ass pocket?
Guy: 375ml of liquor, it fits in your ass pocket.
Girl: Well it's not on UrbanDictionary.
Guy: It is now.
A fifth (200 ml) flat bottle of liquor. So named because it easily slides into the ass pocket. Immortalized on blues legend RL Burnsides' "A Ass Pocket of Whiskey"
Yo, I got an ass pocket of Jameson's, imma go sit under a bridge and get fookinripped.