A bottle of liquor in which you take with you to typically to events in which they usually dont sell liquor or in which the establishment may sell liquor but it's over priced.
Johnny says hey Jim lets go to the cubs game tonight.
Jim says to Johnny well I cant go because I dont have a lot of money.
Johnny says its ok lets just get pocket bottles.
Kim says to Johnny you want to go watch a movie?
Johnny says ok sure but let's get pocket bottles since they dont sell alcohol there.