During the Spring and Autumn period, constant warfare raged among the feudal states. In 589 BC, Duke Qing of Qi attacked the state of Lu, capturing the city of Longyi on Lu's northern border. Lu sent an urgent plea for help to the state of Wei. Duke Mu of Wei dispatched his general Sun Liangfu to lead a rescue force, but the Qi army crushed them at Xinzhu, leaving Wei's troops utterly defeated.
Outnumbered and unable to resist the powerful Qi army, Sun Liangfu fled to the Jin state for reinforcements. Duke Jing of Jin appointed the general Xi Ke as commander of the central army, uniting forces from Lu, Wei, and Cao to launch a counterattack. Xi Ke led the allied army to the foot of Mount Miji (modern Qianfo Mountain in Jinan), where they camped opposite the Qi forces across a ridge.
Arrogant and overconfident, Duke Qing of Qi completely dismissed Xi Ke, sending a challenge letter that very night demanding a decisive battle the next morning. Xi Ke read the letter and sneered at the messenger, "Tell your ruler this: Jin, Lu, and Wei are brother states. Your Qi constantly bullies Lu and Wei. Our lord, unable to bear their suffering, sent me to make you taste the bitterness of being oppressed. Tomorrow, I will crush your Qi army into the ground."
After the messenger returned to camp and reported to Duke Qing of Qi, the duke was so furious that smoke seemed to pour from his seven orifices, saying, "Xi Ke is so arrogant—he's infuriating me to death!"
Duke Qing of Qi had a general named Gao Gu, a battle-hardened warrior of extraordinary strength. Hearing the duke's words, he shouted, "What's so great about Xi Ke's Jin army? I'll go teach him a lesson right now!"
With that, he mounted his swift horse, cracked his whip, and charged straight toward the Jin army camp. As soon as he reached the camp gate, he knocked down two guards with stones, then burst inside, seized a Jin soldier, leaped onto his chariot, and rode back to the Qi lines with his captive.
Gao Gu tied the prisoners to a mulberry tree in front of his camp, drove his chariot in a swift circle through his barracks, and shouted triumphantly, "My courage is not yet spent! Who wants to buy the rest of it?"
Gao Gu's act of valor earned him thunderous applause from the soldiers of Qi's camp.
The next morning, the armies of Qi and Jin clashed in a decisive battle. Thanks to the Jin forces' united fury and skilled command, Qi ultimately suffered defeat. Yet General Gao Gu of Qi, who had daringly raided the enemy camp the night before, was still hailed for his fearless valor.
Later, the idiom "Yu Yong Ke Gu" came to describe someone with exceptional courage or whose strength has not been fully spent, with energy still left to use.
Source: *Zuo Zhuan*, "Duke Cheng's Second Year"
Meaning of the Idiom: Later, the Chinese idiom "余勇可贾" came to describe someone with exceptional courage or whose strength has not been fully spent, with energy still left to use.