When man is getting done in the ass by another man and, selflessly, reaches back around the donator of the cock and fingers his bum hole.

A comical way to describe a woman, as they have a "bum" both at the front and rear.
as used by Jimmy Carr

This phenomenon occurs when a man or woman has a beer belly and s/he wears a belt in the middle creating a bum-like effect.

See photo: This phenomenon occurs when a man or woman has a beer belly and s/he wears a belt in the middle creating a bum-like effect.

The action that occurs when one bum touches another, creating a double bum.