During the Spring and Autumn period, the small states of Zheng and Xi, both surnamed Ji and sharing ancestry with the Zhou royal house, were close neighbors—Zheng was modest in size, but Xi was even smaller.
Though the state of Xi was small, its ruler could not maintain friendly relations with the state of Zheng, and they often engaged in pointless disputes.
In 712 BCE, the ruler of the Xi state, unable to contain his fury over a minor dispute, rashly dispatched his army to attack the neighboring state of Zheng.
The State of Xi had many weaknesses: First, it did not consider whether this military campaign was just; second, it overestimated its own strength, like throwing an egg at a rock; third, it failed to maintain friendly relations with its fraternal states of the same surname; fourth, it could not distinguish right from wrong; fifth, it was blind to its own faults. Thus, from the very beginning, the State of Xi was at a disadvantage.
Tragically, the ruler of Xi remained utterly blind to the truth, stubbornly pressing his army forward—only to suffer a devastating defeat and return in disgrace.
Later, the State of Xi was eventually conquered by the powerful State of Chu. Commenting on Xi's military campaign, later generations said: the ruler of Xi committed "five grave errors" yet persisted obstinately, so his defeat was well deserved.
Later, people used the idiom "defying the world's greatest disapproval" to refer to committing the world's biggest mistake.
Source: *Zuo Zhuan*, Chapter "Duke Yin, Eleventh Year"
Meaning of the Idiom: Later, the Chinese idiom "冒天下之大不韪" came to describe committing the world's biggest mistake.