To fuck a chick with a big pussy. A normal sized pussy is called a camel toe. A pussy with large outer lips is called moose knuckles. Therefore, when you fuck a chick with moose knuckles, your balls slap her enormus lips. So your knuckle balling.
Dude, I hooked up with this hot chick last night but when the panties came off she had the biggest pussy lips I've ever seen. So, did you bolt? I thought about it, but I went knuckle balling instead.