Dong Hu was a historian during the Spring and Autumn period, serving under Duke Ling of Jin
Duke Ling of Jin ascended the throne as a young ruler, naive and cruel. He amused himself by shooting pedestrians from a high terrace with a slingshot, and when a chef failed to cook bear paws to his taste, he ordered the man executed. Prime Minister Zhao Dun repeatedly admonished him, but the duke initially admitted his faults only to ignore them, later growing resentful and attempting to murder Zhao Dun multiple times without success. Fearing for his life, Zhao Dun fled the capital. His cousin Zhao Chuan, seizing the moment when Duke Ling was drunk in a peach garden, had trusted soldiers assassinate him. Upon hearing the news, Zhao Dun rushed back overnight, installed Duke Cheng of Jin as the new ruler, and resumed his role as prime minister, continuing to govern the state.
The historian Dong Hu recorded the event in the official annals, writing: "Zhao Dun murdered his lord." In feudal society, killing a ruler or elder was called "murder" and was considered an act of treason. Zhao Dun quickly explained to Dong Hu that he had not committed the crime of regicide. Dong Hu replied, "You held the position of prime minister. When you fled, you did not leave the state, and when you returned, you did not punish the assassin. If you are not responsible for this crime of killing the lord, who should be?"
Later, people came to call writing that openly and honestly records everyone's faults and shortcomings, without concealment, "Dong Hu's Brush."
Source: *Records of the Grand Historian*, "House of Jin"
Meaning of the Idiom: Later, the Chinese idiom "董狐之笔" came to describe writing that openly and honestly records everyones faults and shortcomings, without concealment.