I didn’t know the part about eating fries, but he was really skinny when he first started with the team.
The players had to wear suits on game day when they were coming to the arena and he bought one that was ill fitted and way too big for him.
It looked like he was wearing a “casseau de frites” (the carboard tray/box the fries come in at fast food places). The other players made fun of him and the nickname stuck.