Etymology: fat + fast food, from one of the common causes of obesity being overconsumption of fast food
Food typically consumed by a specific fat person that is the cause of their weight gain, whether it be fast food or not.
Two teenagers are hungry and want something to eat.
Fat boy: Hey, I'm hungry. Want to get something to eat?
Skinny boy: Sure. Let's go to Chipotle.
Fat boy: Nah, I'm in the mood for McDonald's.
Skinny boy: But you eat so much fat food!! Didn't you just say you were trying to lose weight a minute ago?
Fat boy: Yeah, but I eat McDonald's almost every day?