a) somebody who will never pass you, but lingers way too close behind you.
b) a tail-gater that fluctuates their speed dramatically on the highway just to piss you off or because they're on their cell phone.
c) a tail-gater who might even pass you, but will eventually slow down and tail-gate someone else.

The act of extreme masturbation. You must "knock one out" whilst in close proximity to any of the following; Your mum, a nun, your boss, a member of parliament, George Michael. A person with capabilities to act upon catching you mid self-abuse obvisouly ups the ante. Ejaculation must be reached before your danger wank target comes (no pun intended) to investigate. The higher the chances of being discovered with one's pants down, pulling one's war face is obviously where the danger comes from. The more danger involved the harder (or softer) it is to complete the task in hand (snigger). The more dangerous the better. The chance of being arrested, pummeled by an angry father or having your hand severed by an arab's sabre means that you are a pro "Danger wanker."

A person who creeps forward behind you at a red light, after you pull up a little, even though the light didn't change.

A bumper humper is a man on man action when he would rather place his penis in other mens asses rather then have sex with a women. This could also be used as a phrase to simply call a man gay.

Someone who tailgates

A driver who tails another driver so closely that his/her headlights cannot be seen.

1) an impolite driver who tailgates cars in front of them

2) a person who gets uncomfortably close to ones back side, typically while dancing