[Source] Liu Xiang of the Western Han Dynasty, "Strategies of the Warring States: Chu Strategies IV".
[Meaning] "Wang" means lost; "lao" means the pen for livestock. It is not too late to mend the sheep pen after the sheep have escaped. This idiom is a metaphor for taking remedial measures after a problem occurs to prevent further losses.
[Historical Anecdote]
During the late Warring States period, the state of Chu had declined from its former strength and prosperity. After King Xiang of Chu ascended the throne, he spent his days in pleasure and entertainment, neglecting state affairs, and even appointed the corrupt minister Zilan as his chief minister. After Zilan seized control of the government, the court became corrupt and disordered, and the people suffered terribly. The senior minister Zhuang Xin, witnessing Chu's dire situation, was filled with grief and anguish, unable to eat or sleep in peace. One day, he rushed into the inner palace to remonstrate with King Xiang of Chu. He said to the king: "Your Majesty, Chief Minister Zilan abuses his power, excludes those who disagree with him, and persecutes virtuous officials. If this continues, the state of Chu will be in grave danger!"
At that moment, King Xiang was enjoying himself and, upon seeing Zhuang Xin rush in to reprimand him, flew into a rage. He shouted: "You old fool! Chu is perfectly safe and sound, so why are you uttering such ill-omened words? Get out of here at once!" Returning home, Zhuang Xin reflected on how his well-intentioned advice had been met with such harsh insults from a foolish ruler. Deeply pained, he left in anger, taking his entire family to relocate to the state of Zhao.
Shortly after Zhuang Xin's departure, the state of Qin dispatched the general Bai Qi with a powerful army advancing directly on Chu. The Qin forces came with overwhelming momentum, routing the Chu army, scattering its soldiers and generals, and quickly capturing the capital city of Ying. King Xiang fled in panic, only finding temporary safety in the city of Yangcheng. At this point, King Xiang finally calmed down, recalled Zhuang Xin's loyal advice when he had rushed into the palace, and was filled with remorse. He immediately sent someone to Zhao to bring Zhuang Xin back.
As soon as King Xiang saw Zhuang Xin, he said: "Earlier, I refused to heed your wise and valuable counsel, causing my country to fall into such a disastrous state—truly heartbreaking. Now that things have come to this, what should I do? I beg you to guide me."
Seeing that King Xiang genuinely showed signs of repentance, Zhuang Xin told him a story: "Long ago, there was a man who raised a flock of sheep. One morning, he discovered one sheep was missing. Upon careful inspection, he found a hole in the sheep pen. A wolf had broken in during the night and carried off a sheep. His neighbors all urged him to repair the pen immediately, but he ignored their advice, saying, 'The sheep are already gone; what's the point of fixing the pen now?' The next day, he found another sheep was missing. Regretting that he hadn't heeded his neighbors' warnings, he quickly repaired the pen. After that, no more sheep were taken by wolves."
After finishing the story, Zhuang Xin analyzed the current situation, arguing that although Chu's capital had been captured, as long as King Xiang rallied himself, the state of Chu would not perish. After hearing this, King Xiang followed Zhuang Xin's advice, worked diligently to improve governance, and restored the state's former power and prestige.