When you're eating (usually carelessly) and you spill food on your shirt. Best example is when you're eating chocolate ice cream and it drips on your white shirt (also when you spill a drink on your shirt/pants). Always is an embarrassing moment and you usually don't notice it until someone calls it out.
Cool guy: Dude, check out this really sick Abercrombie & Fitch white polo I just bought.
Friend: Nice! Hey you want some chocolate ice cream on a cone?
Cool guy: Shit yea I do!
(Spills as he eats then goes and chats it up with a hot chick)
Friend: Dude... fail... That's a gnarly Fat Kid Stain on your shirt!
The fat-boy-cut is a type of haircut that would appear on the heads of fat children in the early to mid 90's. Its a bowl cut placed in a high up position on a completely shaven head. The type that rock the fat-boy cut are usually a couple of guys from your old school, normally the ones that you knew who they were but didn't really know them. For visual aid see Zachery Ty Bryan in a pre-puberty episode of Home Improvement.
"Dude, getting punished by Krampus sucked bad but at least he liked my fat-boy-cut."
Guy 1: "HeyTom, give ya the usual?"
Guy 2: "Hmmm... no I think i will just have ya give me the fat-boy-cut."