similar to mud flaps on a truck-butt flaps are a new fashion accessory. They come in plain, name brand, and chrome (popular with the ladies). Can be used to keep the dirt from kicking up while you're running or simpley to just say "Hey, look at my butt flaps!" Hopefully, one day, the famous rapper Little John will come out with his own line of butt flaps.
Gurl, those are some mighty fine butt flaps you got there!