When a fat-endowed person appears to have a buttocks where his belly should be. The belly button resembles a butthole and a deep ravine is created to give the illusion of a buttcrack.
It can also be demonstrated by anyone smooshing together both sides of their belly, which is just goddamn charming.
A portmonteau of belly+buttocks.
"Then she was like, 'Wanna see my ass?' and I was like, 'Fuck yeah,' but she pulled up her top and whipped out her bellocks, which is not the ass I wanted to see."
"Can't believe you fell for that, dude."