In the Eastern Han Dynasty (东汉), there was a man named Chen Shi (陈寔) .
One day, a thief slipped into his living-room. Hiding behind the beam, the thief waited for Chen Shi to fall asleep.
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."
The thief, hearing this, hurriedly jumped down and knelt on the ground to beg forgiveness.
This idiom is used to refer to a thief.