bum knuckle

The lower part of the spine, the coccyx.

I slipped on the ice and landed on my bum knuckle.

by farther jack June 14, 2010

Bum Knuckle

A particularly large irritating haemorrhoid.

the steak Geoff had the night before created such difficulty pushing the poo out that it created a real bum knuckle.

by Dragon Iron January 5, 2014