An ass so large and luscious, that it has it's own gravitational pull. Typically located on an African-American Female. If you come too close to one please beware, you might fall into the event horizon--much like a black hole--and not be able to prevent yourself from grabbing it. This term is a combination of the latin word 'gravitas' (weight), and the word 'ass.'
Tyrone: "DAYUM BRUH! Have you seen dat phat ass on Shaniqua lately?"