It is the opposite of a brown bag special, the opposite of butter face. A girl who is said to be a body bag has a gorgeous face, but the body of a hippopotamus. Blow jobs from body baggers are okay only when drunk.
Dude A: "That girl has such a cute face"
Dude B: "You mean that body bag over there, her body is trash"
Dude A: "I'd still delight in a BJ from her"
Dude B: "Only when you're drunk"
Dude A: "Of course"
Dude A: "That girl is like a reverse paper bag"
Dude B:"She's like a butter body"
Dude C:"You mean a body bag?"