Balls. When sitting correctly, a mans testes may cover the anus and becoming a 'mud flap'. Also, when two are engaged in intercourse in the missionary position, the mans testes may cover the woman's anus and therefore becoming the woman's 'mud flap'.
A mudflap is something that hangs underneath but directly behind the wheels of any land vechicle. Mudflaps are used so that mud doesn't fling up while the wheels are spinning in rocks, dirt and of course, mud.
1. Mud flaps are great additions to all-terrain vehicles.
2. People at Urban-Dictionary don't know the meaning of "Mudflap".