During the Spring and Autumn period, King Fuchai of Wu attacked Yue to avenge his father, crushing the Yue army. King Goujian of Yue was captured by Fuchai and, under the counsel of his advisor Fan Li, feigned surrender, enduring humiliation as a vassal to plot his eventual comeback and revenge.
After King Fuchai of Wu grew arrogant and indulgent in victory, King Goujian of Yue secretly prepared to reclaim his kingdom. In 484 BC, Fuchai planned to attack Qi to dominate the central plains. Goujian brought lavish gifts to pay homage, winning the court's favor. Only the minister Wu Zixu remained wary, warning Fuchai, "Your Majesty, Goujian's humility is a mask—his gifts are poison. Do not trust him."
The king of the state of Wu, Fuchai, was preparing to attack the state of Qi, but his advisor Wu Zixu strongly opposed the plan. Wu Zixu warned, "The state of Yue shares our borders, speaks our language, and shares our customs. They have long harbored ambitions to swallow our land. If we do not destroy them completely, they will eventually destroy us. Conquering Qi would be like gaining a barren, stone-like field that yields nothing. Yue is the true threat to our heart and vitals."
However, King Fuchai of Wu ignored Wu Zixu's warnings and was ultimately defeated by King Goujian of Yue, leading to Fuchai's suicide.
Later, the idiom "Heart and Belly's Affliction" came to describe a serious hidden danger within an organization or department.
Source: *Zuo Zhuan*, Chapter "Duke Ai, Year 11"
Meaning of the Idiom: Later, the Chinese idiom "心腹之患" came to describe a serious hidden danger within an organization or department.