A chin that has a cleft in it giving it the characteristics of a bum. Most people will find the bum chin to be very attractive as it has a certain heir to it. Often associated with power and sophistication. The bum chin is a sign of a distinguished gentleman.
People who have bum chins are more likely to be successful and chosen by the opposite sex, for mating.
John Travolta has an incredible bum chin.
Steve must be successful, just look at that bum chin.