When the People at McDonalds fuck up your order by forgetting your fries, milkshake size, not taking the pickles off your big mac, forgetting your extra patty you ordered, or just plain fucked up your order
Guy 1: Man!! the dumbass at the counter forgot to take off the tomatoes and forgot my fries!!
Guy 2: Haha! Dog, he had a McFuckup on your order hardcore!
People who reside in Mcdonalds for most of their lives. They are nocturnal creatures who looks similar to the inside of a yaks arse. They have a distinct smell of rotten testacles with cheese on toast.
The boys John (fatus) Dalton, Patrick (retard) Quinn and Alistair (Poofter) Lobb are perfect demonstrations of the word McFuckup.