"Musket Feet" derived from a style of snowboard pants. It is known when the lower cut of the pants (normally tighter pants), opens up more than the rest of the pant, much like Bell-Bottoms. The shape of the bottom of the pants creates a 'Musket-like' look to them. Muskets...like the gun.
"Dude check that whack style that bro has, he's got some mad Musket Feet."