During the Spring and Autumn period, King Gong of Chu planned to attack the state of Zheng. Zisi, the son of Duke Mu of Zheng, prepared to go to Chu to negotiate a peace alliance.
After Zi Kong and Zi Zhi learned of this, they said to Zi Si, "Our state of Zheng has just formed an alliance with Jin; the blood wine from the oath is still wet on our lips—how can we break the covenant and ally with Chu instead?"
Zi Si smiled and said, "Our covenant with the state of Jin was clear: Zheng only submits to the powerful. Now Chu's army attacks us, and Jin offers no aid—this proves Chu is the stronger state. If we ally with Chu, how can that be called betraying our oath to Jin?"
Zi Kong and Zi said, "Your words make sense, but if we do this, the gods will be displeased."
Zi Si disagreed, saying, "The gods only honor oaths made in good faith. Our alliance with Jin was made under duress, so it holds no true faith. Breaking it will not incur divine wrath."
Zikong and Zili nodded in agreement.
After arriving in Chu, Zisi expressed to King Gong of Chu that the state of Zheng was willing to form an alliance and make peace. King Gong agreed and brought Prince Barong with him to Zheng to hold the alliance ceremony. However, before the ceremony could stabilize Zheng, King Gong's mother, Lady Zhuang of Chu, passed away, forcing him to return home for the funeral.
Later, the idiom "lips still wet with blood" came to describe breaking an oath soon after making it.
Source: *Zuo Zhuan*, "Duke Xiang's Ninth Year"
Meaning of the Idiom: Later, the Chinese idiom "口血未干" came to describe breaking an oath soon after making it.