Mei Cheng was a renowned poet and rhapsodist of the Western Han Dynasty.
In the early years of the Western Han Dynasty, Emperor Gaozu Liu Bang enfeoffed many princes of the same surname, and his nephew Liu Bi was granted the title of King of Wu in the Wu region.
After Emperor Gaozu of Han died, Emperor Jing of Han ascended the throne and heavily relied on Chao Cuo, who advocated weakening the power of the vassal kings. The King of Wu, Liu Bi, allied with the seven states of Chu, Zhao, Jiaodong, Jiaoxi, Jinan, and Zichuan, plotting rebellion under the pretext of "executing Chao Cuo to clear the side of the ruler."
At that time, Mei Cheng was serving as a court gentleman under Liu Bi, the King of Wu. Upon learning of Liu Bi's plans to rebel, Mei Cheng wrote a memorial titled "Admonishing the King of Wu," urging Liu Bi not to start a rebellion, warning that doing so would lead to his own destruction.
In his "Letter to the King of Wu," Mei Cheng advised the king against rebellion. Since the king's plan was not yet public, Mei Cheng could only use metaphors to hint at his warning. He wrote:
"The situation is extremely critical. It is as if a heavy weight of a thousand jun (one jun equals 30 jin, a thousand jun equals 30,000 jin) is hanging from a single strand of hair—truly perilous. Now, the nation stands at a life-or-death moment. If you heed the counsel of loyal ministers, the country will be fortunate indeed."
However, Liu Pi remained stubborn and soon launched his rebellion. Seeing that Liu Pi ignored his advice, Mei Cheng defected to Emperor Jing's brother, Prince Xiao of Liang, and wrote another letter, "A Second Admonition to the King of Wu," but Liu Pi still refused to listen.
When Emperor Jing of Han learned that the seven kingdoms of Wu and Chu had rebelled, he dispatched the great general Zhou Yafu to lead the imperial army to suppress the uprising, and the rebellion was swiftly quelled, with all seven rebel kings either killed or forced to commit suicide.
Later, the idiom "A Thousand Jun Hanging by a Single Hair" came to be used to describe extreme danger or a critical situation.
Source: *Book of Han*, "Biography of Mei Cheng"
Meaning of the Idiom: Later, the Chinese idiom "一发千钧" came to describe extreme danger or a critical situation.