So it's a deliberate form of hungry bum then....?

'Hungry bum' being an affliction that affects most people at some time of their lives. The underwear is gathered into the butt crack by the action of walking, and forms an uncomfortable wadge, removed by the action of 'arse picking', like nose picking but at the other end. The effect is as if the arse had decided to be the top end of the digestive tract for a change.

It sometimes occurs when the elastic becomes saggy and loose, or if a particular pair of trousers of synthetic material are worn with chuddies of a similar fabric.

LTS