A popular word used by people with colorful and inside humor. "McFucking" does not have anything to do with McDonald's, It's simply a cute way of saying "fucking".
When someone is so ugly that they simply cannot be described with actual words, so one decides to simply say "mufuckin uhhh" in place of an adjective to describe their vulgar appearance.
"Caleb who let you in the picture looking like that"
"Like what?"
"A mufuckin uhhh"