Ran away with a stolen bucket of chicken like in "precious"

He ate the evidence.

I think he did something particularly heinous. Rape/murder. But because he is a full fledged psychopath he really believes he didn't belong there. And his beating/death showed Andy that no matter how hard you try to explain yourself, nobody there gives a crap about you or your story.
We are the music makers and we are the dreamers of dreams. -Willy Wonka

Did it ever come up? I doubt his offence was mentioned. He just annoyed the sadistic warder who beat him up for the fun of it.

Indecent exposure. He mooned someone with that big mooshy butt of his.

Probably a child molester.

Fatass is Heywoods buddy. Snuck on the bus to help Heywood win the bet.

I've always played it out in my head that he truly wasn't supposed to be there. That by some clerical mistake, or identity switch, he ended up in the wrong situation. I suppose it was the sincerity with which the character is played. I've often wondered if there was ever disposed of scenes that showed him getting screwed around by accident and ending up where he did. But then minds were changed as it would be too "on the nose" with Andy's story, so they just kept the breakdown and beating.

Probably breaking into McDonald's.
Schrodinger's cat walks into a bar and doesn't.

I know exactly what the fat guy did to end up in prison.
He stole three and a half donuts from a donut shop.
The shopkeeper, who was wearing a red shirt and black pants,
grabbed the fat guy, and called the police immediately.
He was arrested, and sentenced for 2 years in a state penitentiary.
But, unfortunately for him, he panicked, and this resulted in his death.

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You mean the fat tub of monkey spunk?
Whose idea was it for the word "Lisp" to have an "S" in it?

He looked like a white collar criminal to me.

I instantly got child molester from him.
Wendy? Darling? Light, of my life!

Cannibalism

Maybe FatAss is a parallel to Andy. At the end of the movie we learn that Andy was wrongly convicted, but ultimately gets his freedom, and revenge on the prison guards and the Warden. But at the start of the movie, during the "fresh fish" scene, were FatAss is pleading with Captain Hadley, he (FatAss) begs that hes not supposed to be here. Not him. So it is possible that FatAss is another wrongly convicted prisoner but, unlike Andy, never got his freedom.

Most likely was having having one of his typical nights raiding McDonald's and Dunkin'Donuts so he can be corpulent enough before hibernation to survive yet another winter.
The Lord is my shepherd, I lack nothing.

Since he seems weak, maybe he went along with friends on a joy ride who stole a car or was talked into being the getaway driver during a bank robbery.
He could also have been framed or as the fall guy for someone else who committed the crime.
Damn, I'm good.

Exactly. It's funny, sometimes people who think they are smarter than others are the beep dumbest out of everyone.

TxMike, since you seem to have missed the point of the question, let me explain. You are right that there is no reason given in the movie and that character exists within the story as a means of fulfilling a narrative purpose within our world. But there is still an "in universe" reason for his being there. When the characters ask each other why they are there, for instance, they admit the crime they committed (Red) or claim they are innocent (everyone else). No one says they are simply there or acts as though any other inmate is simply there. When there is no real world explanation for something within a story that people enjoy, part of the fun is in contemplating what the in universe explanation for those things might be. That was the intent of the original question.

Ha-ha you wish you were as "retarded" as I am. Care to compare accomplishments?
.... TxMike ....
Sometimes I think we're alone in the universe, and sometimes not.

TxMike smart??? The guy is renowned for being the biggest retard on IMDb. Nobody pays attention to him!
24/04/1916

Perhaps, but unlike you they don't act like arrogant pricks thinking they're better than others because they're smarter.
"A real friend or mentor is not on your payroll."Prince, 1958-2016

Have you been bothered all your life by someone who actually has an intellect? Who actually thinks about things before responding? Most of you kids here seem brain-dead.
.... TxMike ....
Sometimes I think we're alone in the universe, and sometimes not.