An utterance made in response to someone believing something terrible but obviously false. Derived from the 'Seriously McDonalds' tweet of a fake racist sign on the door of a McDonalds.
Jake: "So my cousin met this chick, and got drunk, and he woke up in a bath tub full of ice missing a kidney!"
Sally: "Oh! That's terrible! Will he be OK?"
Max (to Sally): "Seriously McDonalds!"