A person so hideous they are not just fugly, they have been elevated to the highest tier of ugliness. It is a top echelon of raunchiness, no other word can fill. Only to be used as a last ditch effort to describe someone.
"There's the good, the bad, the fugly, and then the dreaded, muthafugly."
"That chick is so muthafugly, my eyes burn."