Jimmy’s talent in stealing began when he was young, when his dad Charles ran a small corner shop. We see in a flashback that Jimmy worked alongside his dad at the shop, and his dad had a reputation of being gullible. This led to their shop being a frequent target to scam artists who easily preyed on Charles' naivety and easily swindled money out of him.
Seeing his dad repeatedly cheated time after time led Jimmy to steal from the till himself, after one of the scam artists offered him a lesson about people being wolves and sheep. Over the years, Jimmy stole over $14,000 straight from the till, which eventually led to its closure. Charles died six months after the shop closed and Chuck recalled that no one cried harder at the funeral than jimmy.
Worryingly, Jimmy would eventually get over his guilt and show little remorse over his dad losing the shop. He blamed his dad for being too generous and that this led to the closing of the shop and ignores the consequence of his own actions.