A Kotflügel, literally "mud wing," refers to the fender - that part of a car covering the wheels. While the Kotflügel may help cars get off the ground in 100 years or so, it currently only offers protection from some rather unpleasant flying substances.

The first fenders were built for horse carriages and acted as a shield - less against mud than against, well, excrement. The mud on the roads didn't just come from horse manure, but also from human feces, which in those times was simply poured onto the streets.

The fenders were especially useful for protecting people in the carriage in front from the muck splashing out from under the wheels.

Fenders basically serve the same purpose today. Even though manure isn't as common on our roads, the Kotflügel is there to make sure pedestrians aren't sprayed by water or mud when a car passes by.