暗箭伤人 — “To harm someone secretly or by underhanded means”

During the Spring and Autumn period, Duke Zhuang of Zheng, with the support of the states of Lu and Qi, planned to launch an expedition against the state of Xu. (Xu was a small state, located in present-day Xuchang, Henan. Zheng was to the north of Xu, with its capital in present-day Xinzheng, Henan.) This event is recorded in the 'Zuo Zhuan' (Annals of Zuo), Duke Yin, Year 11.

That summer, in the fifth month, Duke Zhuang reviewed his troops and distributed chariots in front of his palace. An old general, Ying Shū Kǎo, and a young general, Gōng Sūn Zǐ Dū, got into an argument over who should take a particular chariot. Ying Shū Kǎo, a brave general, refused to be seen as old; he pulled the chariot and quickly rode off. Gōng Sūn Zǐ Dū, who always looked down on others, naturally refused to yield. He drew his long halberd and chased after him. By the time he reached the main road, Ying Shū Kǎo was already out of sight. Gōng Sūn Zǐ Dū harbored a deep resentment because of this incident.

In autumn, in the seventh month, Duke Zhuang of Zheng officially commanded the attack on Xu. The Zheng army approached the capital of Xu. During the siege, Ying Shū Kǎo bravely led the charge and was the first to climb the city wall. Gōng Sūn Zǐ Dū, seeing that Ying Shū Kǎo was about to achieve a great feat, grew even more jealous. He drew an arrow, aimed it at Ying Shū Kǎo, and shot him. The brave old general fell head over heels from the wall. Another general, Xiá Shū Yíng, mistakenly thought Ying Shū Kǎo had been killed by the Xu soldiers. He quickly picked up the battle flag and commanded his troops to continue fighting, eventually breaking through the city. The entire Zheng army entered the city, and the ruler of Xu, Duke Zhuang of Xu, fled to the state of Wei. The territory of Xu was then incorporated into Zheng's domain.

Such an act, like Gōng Sūn Zǐ Dū's secretly shooting an arrow when others were unprepared, is called '暗箭伤人' (ànyǐnjìan shāngrén). However, as an idiom, its meaning is not limited to using a secret arrow as a weapon. It refers to any situation where one secretly harms others by any underhanded or treacherous means.