lump ass Share definition Flag When a girl has a wallet, phone, pack of cigarettes, etc. in the back pocket of her tight-fitting jeans, thus causing an odd protrusion from her butt. Jamie: Man, if that girl is gonna wear such tight jeans she better upgrade to a smaller phone! Christina: Yeah, shes got some seriousss lump ass! lump ass by Nathan T April 1, 2008 Get the lump ass mug.