一发千钧 — “In imminent peril”

Han Yu, also known by his courtesy name Tuzhi, was a prominent literary figure from Nanyang, Dengzhou during the Tang Dynasty. He championed the idea of "literature conveying the Tao" and advocated for a return to classical prose, replacing the ornate parallel prose style. His influence on his contemporaries and future generations was immense, earning him the reputation of having "revived prose after eight dynasties of decline."

He was a staunch opponent of Buddhism. When Emperor Xianzong dispatched envoys to welcome Buddha relics into the court, Han Yu submitted a memorial vehemently objecting to it. This act offended the emperor, leading to his demotion and banishment to Chaozhou as a governor.

In Chaozhou, Han Yu became acquainted with an elderly monk who was intelligent and understanding. As Han Yu had few friends in Chaozhou, he developed a close relationship with this monk. Consequently, rumors began to circulate that Han Yu had also converted to Buddhism.

His friend Meng Jidao (Meng Jiao), who was then a minister and a devout Buddhist, was also banished to Jizhou after similarly offending Emperor Xianzong. Upon arriving in Jizhou, he too heard the rumors about Han Yu's supposed conversion. He felt somewhat perplexed, knowing Han Yu's strong opposition to the faith. Therefore, he specifically wrote a letter to Han Yu to inquire about it.

Upon receiving Meng Jidao's letter, Han Yu realized that his interactions with the monk had led to this misunderstanding. He immediately wrote back to Meng Jidao to clarify the situation. Furthermore, Han Yu sharply criticized the court officials of his time who embraced Buddhism, neglected Confucian principles, and used superstition to delude the emperor. He also expressed great indignation at the emperor's alienation of worthy individuals and the decline of Confucianism.

In his letter, Han Yu included a passage that read: "百孔千疮，随乱随失，共危如一发引千钧…" (Full of holes and scars, lost in chaos, as perilous as a thousand jun pulled by a single hair...). This vividly depicted a situation of extreme danger, like a thousand jun (an ancient unit of weight, equivalent to about 30 catties or 15 kg per jun) hanging by a single strand of hair. Today, people often use this idiom to describe any situation that is critically perilous. The idiom "一发千钧" originates from Han Yu's letter to Meng Shangshu, containing the line: "共危如一发引千钧，绵绵延延，洼以微灭."