(n) An ass so large as to simulate a shelf, table, nightstand, or other flat tabular object that one might set a drink, ash tray, plates, a TV, or what not on.
"Bitch got a shelf butt that you could stack an encyclopedia on!"
by Bystrick March 22, 2003
A large butt on an otherwise not large woman. So oddly large that it protrudes like a shelf.
If you just saw Jane from the waist up, you'd never guess how big her ass is ... what a shelf butt!
by BWS June 06, 2005