to pop like a poppy seed, or, the act of

a big backside; junk in da trunk

Bits of ugly fat hanging over the top of hipster jeans when someone squeezes into a size too small.
Most visible from behind, the unsighly bulges can be spotted in almost any pub or night club in the UK on a Friday or Saturday night, and sightings have been reported during the daytime too.
The hipster jeans are usually accompanied by a tight t-shirt which rides up to the narrower part of the body due to the width of the plaps, making them more obvious. The t-shirt may also be a size too small, although this is not always the case.