n. One who packs any object into his or any other mans rectal cavity. Generally one that is called an "ass packer" is known for engaging in intercourse with another male (i.e. fucking him up the ass) although the penis-to-anus contact is not necessary for one to be dubbed an "ass packer"
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.