1. A "fender bender" is a minor automobile accident, one which causes damage but not injury, i.e. "it bends the fenders" (but more commonly the bumpers and lights).Of course, the term originated when fenders were always sheet metal that might be bent back into shape. This is not the case with plastics and fiberglass.

2. A fender bender is simply when a guy gets a 10 to 15 foot running start to rail a girl in the back door while she is bending over; Making an obvious
reference to the car accident when one vehicle crashes into the ass end of the vehicle in front of it.

www.urbandictionary.com/define.php?term=fender+bender