Someone who does not respect the safe and proper distance between automobiles. Known as a
(1) You are driving on the highway, in the slow lane, and your cruise is set at 55. There is no one around. A car gets on the highway behind you and quickly accelerates to 65. He is soon on your tail, and could easily pass you, but instead he rides your ass for the next 5 miles. What a bumperfucker! Probably a 35 year old man in a large SUV.
(2) You are driving on the highway, in the slow lane, and your cruise is set at 55. There are lots of cars whizzing past you in the fast lane. There is no one behind you for half a mile. You are coming up to a green light at an intersection. Then some "possum" who is sitting at the red light perpendicular to you decides to make a right turn onto the highway directly in front of you traveling at 5 mph. This is a (reverse) bumperfucker. Probably an 80 year old woman in a Buick.
Someone who drives too close to your rear bumper, or someone who gets in front of you only to go slower than you.
I feel like driving a tank and squashing all these bumperfuckers!