It's probably from a fight. There's also a scar on the back of his head by one of his ears, isn't there? He was expelled from every school he ever been to probably for fighting.
Then again, Jimmy comes from a hard homelife. Like Merilyn suggested, maybe abuse? Good Lord I hope not! That would be so depressing
I don't know if that wouldn't really make sense though only because you don't know too much about Jimmy's homelife. The only thing you really know for sure is that he fights all the time!