During the Western Han Dynasty, there was a stern and upright official named Yan Yannian.
During the reign of Emperor Xuan of the Han Dynasty, a man was first appointed as the governor of Zhuo Commandery. At that time, the local powerful clans were extremely formidable, with the Eastern Gao and Western Gao families being the most vicious, committing every evil and arousing immense public outrage.
Yan Yannian quickly learned of the situation and sent his subordinate Zhao Xiu to investigate the crimes of the Dong and Xi clans. Zhao Xiu, afraid to offend the powerful families, concealed their offenses and filed no report. Yan Yannian immediately executed Zhao Xiu for dereliction of duty. Then, after confirming the charges against the Dong and Xi clans, Yan Yannian arrested them and beheaded the ringleaders.
The astonishing news spread throughout the entire commandery, with common folk rushing to tell one another and clapping their hands in delight. The arrogance of the remaining local tyrants greatly subsided, and Zhuo Commandery was brought to order.
Three years later, Yan Yannian was promoted to Grand Administrator of Henan, a major commandery teeming with powerful clans like the Eastern Gao and Western Gao families. Upon his arrival, an advisor warned him, "Go easy on them, or you'll make enemies." But Yan Yannian replied without hesitation, "I fear no one," and proceeded to crack down ruthlessly on lawless magnates while fiercely protecting the poor and weak.
Yan Yannian handled cases differently from other officials: when the poor and weak committed crimes, he often showed leniency based on circumstances, but when wealthy and powerful families bullied commoners, he struck them down harshly. Neither officials nor commoners could fathom his intentions, finding him unfathomable and thus too terrified to break the law. The Henan Commandery was once again well-governed.
But because Yan Yannian enforced the law strictly and executed many powerful aristocratic families, he incurred their hatred. They falsely accused him, leading to his execution by punishment.
Later, the idiom "unfathomable depth" came to describe someone whose thoughts are hard to gauge, and also to mock those who deliberately mystify.
Source: *Book of Han*, "Biography of Li Yannian"
Meaning of the Idiom: Later, the Chinese idiom "莫测高深" came to describe how someone whose thoughts are hard to gauge, and also to mock those who deliberately mystify.