A man heard that Guan Yu was injured and came to the rescue. That man was Hua Tuo. Hua tuo cut the meat with a sharp knife until the bone turned blue, so scraping the bone cured Guan Yu.

Scraping the bone to treat poison is to scrape off the venom that goes deep into the bone with a knife to achieve the purpose of treatment. Especially in the historical novel Romance of the Three Kingdoms, Hua Tuo cured Guan Yu by scraping bones. Later, he also used scraping bones to treat poison as a metaphor for a strong-willed person. It also means strong will.

Source: The History of the Three Kingdoms by Chen Shou in the Western Jin Dynasty: Yu then stretched out his arm and ordered the doctor to cut it. The feather suit was smooth, so the general was invited to eat and drink, and the blood flowed from his arm, full of dishes, and the feather suit made him laugh.

Interpretation: Guan Yu stretched out his arm and asked the doctor to cut it open. At that time, Guan Yu was sitting around drinking with people, his arm was dripping with blood, and his plate was full of blood, but Guan Yu was cutting meat and drinking, talking and laughing as usual.