One who drives closely on another's bumper to intimidate such that the closely followed/bumper-bullied will change lanes (from fast lane to slow) to enable the subsequent bumper-bullying of those in front of him/her who might be merely driving the speed limit.
Bob was enjoying his coffee on his commute to Denver from Fort Collins, and though he was driving the maximum speed limit his rear-view and side mirrors served him a constant reminder of a SUV bumper-bullying him to change lanes.
A belly bumper is a person who is unmotivated, lazy and/or lacks any sense of completion. A person who possesses this type of behavior can partake in said belly bumping activities.
A belly bump is also the clear act of two belly bumpers uniting!
That guy just belly bumped his way through work today!