During the Spring and Autumn period, after Duke Xiang of Jin passed away, his infant son Yigao was the rightful heir but too young to rule. Zhao Dun, the powerful commander of Jin's central army, seized control and proposed installing Prince Yong instead. He told the ministers, "Prince Yong is older, our late lord favored him, and he has close ties with the state of Qin—making him ruler would bring stability to Jin."
Minister Jia Ji opposed the appointment of Prince Yong and instead advocated for Prince Le.
Zhao Dun ignored all advice and sent men to Chen to assassinate Prince Le, then dispatched others to Qin to welcome Prince Yong back to the state.
Lady Mu Ying, the wife of Duke Xiang of Jin, upon hearing this news, came to court every day carrying the crown prince Yigao, weeping and saying, "What crime did the late ruler commit? What crime did his heir commit? To abandon the legitimate son and seek a ruler from outside—how do you intend to place this crown prince?"
After leaving the court, Mu Ying carried the young crown prince to Zhao Dun's home, knelt before him, and said, "The late king entrusted this child to you with these words: 'If he grows into a worthy ruler, I will have received your gift; if not, I will hold you responsible.' Now the king has passed, but his words still echo in my ears. Why have you abandoned them?"
Zhao Dun and the high ministers, fearing that Mu Ying would cause trouble and worried that other princes might oppose them, changed their minds and installed Yigao as the ruler of the state of Jin.
By this time, the state of Qin had already dispatched troops to escort Prince Yong back to Jin. To quell the conflict, Zhao Dun immediately launched an attack on the Qin forces, defeating them at the battle of Linghu.
Soon after, Di troops invaded Lu's western border, and Duke Wen of Lu sent an envoy to report to Jin. Zhao Dun dispatched Jia Ji to the Di state to meet their chancellor Feng Shu, reprimanding him for the attack on Lu.
Feng Shu asked Jia Ji, "Between Zhao Cui and Zhao Dun, which one is more virtuous?"
Zhao Shuai was a senior minister of the Jin state, wielding military and political power. After his death, his son Zhao Dun succeeded him. Because Zhao Dun rejected Jia Ji's advice to install Prince Le as ruler and instead had Prince Le killed, Jia Ji came to view Zhao Dun as autocratic and ruthless.
Jia Ji replied, "Zhao Shuai is the winter sun, Zhao Dun is the summer sun."
The winter sun is warm and lovely, but the scorching summer sun is fearsome.
Later, the idiom "Xia Ri Ke Wei" (Summer Sun is Dreadful) came to describe someone stern and unapproachable.
Source: *Zuo Zhuan*, "Duke Wen's Seventh Year"
Meaning of the Idiom: Later, the Chinese idiom "夏日可畏" came to describe someone stern and unapproachable.