There was a minister named Li Yifu during the Tang Dynasty (618-907). He was good at flattering people. So his official position was raised higher and higher. At last, he was made the prime minister. He was a vicious man. When he talked with people he always wore a sweet smile, but he was conjuring up evil ideas in his heart.
Once Li heard that a beautiful woman was put in prison, he sent for the prison officer and ordered him to set the woman free. The man followed his order and Li took the woman home.
Later, someone reported the case to the emperor. The prison officer was very afraid because he knew he had committed a crime by setting the criminal free. He had to go to Li Yifu for help. But Li ignored the thing and asked the man not to disturb him. The poor man was so disappointed that he hanged himself.
Another officer heard the tragic suicide of the prison officer, wanted to reveal the truth to the emperor. But Li knew his plan and he began making false accusations against him. The foolish emperor believed Li's words and exiled the officer to a distant area.
Later, the idiom has been used to describe a man with murderous intent behind his smile.