1) The area around the anus where the skin is darker in color than the rest of the body. The mudring if often bleaced by porn stars to make it look nonexistent and thus make the asshole look more enticing.
2) The area around the balloon knot.
My brother has the biggest, dirtiest mudring I've ever seen.
A Mud Ring is the perfect circle formed in a grown ass mans underpants after he sharts.
After Jack sharted he threw his undies in the laundry hamper, Cindy went to take care of the laundry then asked Jack is that a mudring on your underpants.. Cindy then asks Jack why is the mud so thick,
the dirty orifice used by many for sexual gratification. also used for excretion of feces.
"I banged that whore in the mudring."
"I just took a huge dump and my mudring is throbbing."
or the ever friendly "she toungue jacked my mudring"