Rather than an idiom, it is more appropriate to say that this is a saying or a folk proverb. For a long time, I have always thought that this sentence was first spread in the Qing Dynasty or the Republic of China, because It is very simple. What I never expected is that this sentence has been circulated in society as late as the end of the Qin Dynasty. This sentence was first seen in "Historical Records: The Liuhou Family", when Pei Gong and Liu Bang's 100,000 rebel troops broke through the Yaoguan Pass. Defeating the Qin Dynasty's Guanzhong garrison at Lantian, leading the world's rebels, and successfully entered Xianyang. After entering the Qin Dynasty's Xianyang Palace, Peigong Liu Bang was attracted by the beautiful treasures in the palace. He forgot about himself and planned to stay in the Qin Palace to have fun. His brother-in-law Fan Kuai tried to persuade him, but he refused to listen. At this time, Zhang Liang went to the palace to directly admonish Liu Bang, and gave Liu Bang a severe scolding, explaining his mistake in doing so, and said: "Poison Bitter words are good for illness, and loyal words that are offensive to ears are good for deeds. Please listen to Fan Kuai, Duke of Pei." Then Pei Gong reluctantly left the Qin Palace and announced that the army would leave the city and stationed obediently in Bashang on the outskirts of Xianyang, without disturbing the people. This is probably this famous saying. This is the earliest source that can be seen so far. The "poison" in the original sentence does not refer to poisonous medicine. "Poison" means "poisonous mouth", which means it tastes very bad.