Refers to either someone with an unusually large ass or the ass itself. Saying made popular by the character 'Frank Rizzo' as played by Johnny Brennan of the comedy team, 'The Jerky Boys'.
Guy 1: Dude, look at that girl's ass!
Guy 2: Wow, she has a barrel ass!
Guy 1: Damn, that girl is fat!
Guy 2: Yeah, she's a barrel ass!
Much like barrel, a barrel ass is a woman who has a massive ass, which resembles a barrel.
Different from a ghetto booty, as there is nothing good about a woman who is considered to be a barrel ass
I cant BELIEVE you hooked up with that barrel ass last night, bitch had to turn sideways to walk out the door!