1. This practice is done for breeding horses. It is the act of lightly scrubing a horses labia with a sterile solution using the knuckles on your hand.
2. The act of pleasuring a woman with your hands.
1. "This mare is ripe for breeding, but first we'd better do a quick knuckle wash. F'shizzle my Nizzle"
2. "The ho really enjoyed the knuckle wash I gave her. F'shizzle my Nizzle"