Mud Flaps Ask Urban Dictionary about Mud Flaps Share definition Flag When someone's butt hangs down pretty low. "Big bottoms, big bottoms! Talk about mud flaps, my girl's got 'em!" Mud Flaps by thatgirldrinkinglemonade July 9, 2003 0 0 Mud Flaps Merch Get the Mud Flaps mug. Buy Now — $32.95 See more merch