Close cousin to the cameltoe, its the outward migration of meaty mudflaps upon introduction of fabric to the crotch of your typical 'split-tail'. Actually very common in the Midwest, most likely to be found at your local club on 80's Night.
When your pants are pulled up so far up that it causes the crotch seamin the pants to split the ballsack in half, therefore giving the appearance of a mooseknuckle.
Also a term used when insulting someone, mainly a tryhard.