In 468 BC, the Duke of Jin sent his minister Xun Yao as commander to attack the state of Zheng. Unable to withstand the Jin army's assault, Zheng dispatched a messenger to Qi for aid.
Duke Ping of Qi could not bear to watch the state of Zheng fall, so he immediately dispatched his minister Chen Chengzi to lead troops to its rescue.
Led by their guide Zisi, the Qi army crossed the Pu River in heavy rain, preparing to engage the Jin army. Jin commander Xun Yao, seeing the Qi forces well-ordered and high-spirited, grew uneasy. He told his generals, "The Qi army is disciplined and formidable—we may struggle to defeat them."
The generals, also afraid to fight the Qi army, advocated for retreat. Xun Yao, while preparing to withdraw, sent an envoy to the Qi camp to meet Chen Chengzi, sowing discord by saying:
"General Chen, you are a descendant of the State of Chen. Though your state was destroyed by Chu, Zheng bears undeniable responsibility. Thus, when our Jin army now attacks Zheng, it is truly to avenge Chen!"
Chen Chengzi said coldly, "I am now a great general of Qi, sent by my lord's command to rescue Zheng. Tell Xun Yao to stop wasting his efforts."
After the Jin envoy left, a general named Xun Yin reported to Chen Chengzi, "I hear the Jin army plans to launch a thousand chariots against us, aiming to destroy us—I fear we cannot defeat them."
Chen Chengzi said sternly, "We have been ordered by our ruler to rescue the state of Zheng. Even if the Jin army is larger, we will not fear. The two armies are about to clash—do you know the enemy's troop deployment, battle strategy, and morale?"
"I don't know!" Xun Yin said. "Since you know nothing at all and only rely on hearsay to boost the enemy's morale and dampen our own spirit, that's truly inexcusable!"
Xun Yin realized his mistake and felt deeply regretful. He said, "A gentleman, when planning a matter, should consider all its aspects thoroughly before reporting upward. But I had no clear idea—how could I not run into a wall?"
As it turned out, the rumors Xun Yin had heard were completely baseless
Later, the idiom "to answer 'don't know' to every question" came to mean being completely ignorant of the actual situation.
Source: *Zuo Zhuan*, "Duke Ai's Twenty-Seventh Year"
Meaning of the Idiom: Later, the Chinese idiom "一问三不知" came to describe being completely ignorant of the actual situation.