[Explanation]: A metaphor for secretly harming someone or employing underhanded schemes.
During the Spring and Autumn period, Duke Zhuang of the state of Zheng, with the support of the states of Lu and Qi, planned to launch a campaign against the state of Xu. (Xu was a small state located in present-day Xuchang City, Henan Province. Zheng was situated to the north of Xu, and its capital at the time was Xinzheng, also in modern Henan.)
This incident is recorded in the "Zuo Zhuan · Eleventh Year of Duke Yin." That summer, in the fifth month, Duke Zhuang of Zheng inspected his troops and assigned chariots in front of the palace. An old general, Ying Shu-kao, and a young general, Gong Sun-zi-du, quarreled over a chariot. Ying Shu-kao, a valiant warrior who refused to acknowledge his age, seized the chariot and quickly drove away. Gong Sun-zi-du, who habitually looked down on others, naturally refused to yield and drew his long halberd, sprinting after him. By the time Gong Sun-zi-du reached the main road, Ying Shu-kao had already vanished. This incident filled Gong Sun-zi-du with resentment.
Later that autumn, in the seventh month, Duke Zhuang formally ordered the attack on Xu. As the Zheng army approached the capital of Xu and began the assault, Ying Shu-kao bravely led the charge, scaling the city walls. Seeing that Ying Shu-kao was about to achieve great merit, Gong Sun-zi-du's jealousy intensified. He drew an arrow, took aim at Ying Shu-kao, and shot. The brave old general tumbled from the wall. Another general, Xia Shu-ying, mistakenly believed Ying Shu-kao had been killed by the forces of Xu. He quickly seized the banner and directed the soldiers to continue fighting, ultimately breaching the city. The entire Zheng army entered the city, and the ruler of Xu, Duke Zhuang of Xu, fled to the state of Wei. Xu's territory was thus incorporated into Zheng's domain.
Shooting a hidden arrow like Gong Sun-zi-du did, taking advantage of someone's unguarded moment, is called "An Jian Shang Ren" (ambush attack). However, as an idiom, it is not limited to using arrows as weapons; any act of secretly harming someone through underhanded or unfair means can be described as "An Jian Shang Ren."
To illustrate further, here is a story about Liu Bin, a man from the Song Dynasty.
Liu Bin, styled Gong-fu, was said to be quite talented in literature, particularly skilled in historical research. He participated in the compilation of the "Zizhi Tongjian," edited by Sima Guang. In the "Hou Lu of Wen Jian" by Shao Bo of the Song Dynasty, there is an anecdote about him, suggesting he also possessed a considerable sense of humor. The story goes: Once, a "Zhong Si" (an official title, meaning "Imperial Censor-in-Chief," responsible for impeachment and supervision) wanted to incite his colleagues to attack a certain person. Someone asked Liu Bin whether this person truly had any serious faults, warranting such a public condemnation. Liu Bin coldly replied, "The Zhong Si may certainly beat his drum; this old man cannot be an 'ambush arrow'!"
Here, the term "ambush arrow" refers precisely to the kind of hidden, malicious attack. Liu Bin refused to participate in such a conspiracy to secretly harm someone, hence his declaration that he would not serve as an "ambush arrow." (For the allusion regarding "beating the drum," see "Beat the Drum and Attack.")