Any type of fast food. Fat is the long abbreviation of fast in "fast food" which makes it fat food... also the fact that fast food is usually extremely fattening, it works perfectly.
person 1- "I'm goin out, you want anything on my way back"
person 2- "Yeah get me some fat food, what you think!"
person 1- "K, what you want"
person 2- " BK nigga! or wendy's"
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?