In the Eastern Han Dynasty (东汉), there was a man named Chen Shi (陈寔) .
Chen Shi, finding the thief, called his family together, saying, "To be a upright man one must have aspirations. Men are not born evil. But if one gets used to doing evil things, it will be hard to reform. The gentleman above the beam is such a man."
This idiom is used to refer to a thief.