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English Translation

lit.: A bird startled at the mere twang of a bow-string
fig.: someone who frightens easily, due to past experiences

Breaking it down

1. 惊: startled, shocked
2. 弓: bow
3. 之: possessive particle (like "of")
4. 鸟: bird

Source: Google Image Search

Story in Chinese

战国时期，有个杰出的弓箭手，叫做更赢。他的射箭本领在当时可称是举世无双。
有一天，他和魏王并肩站着，天空中忽然飞过一群鸿雁。更赢很自信的对魏王说：“我可以用弓声就把飞鸟给打下来。”魏王很怀疑。
正在那是，一只孤雁很低很慢的飞过。鸣声凄惨。更赢见了，就张着弓，扣着弦，砰的一声，直入云宵。那孤雁果然应声落地。魏王惊叹之余，不明白这是怎么回事。 更赢解释说：“那孤雁飞得低且慢，因为它已经受过伤；它鸣叫的声音悲而哀，因为它离了群。身伤心碎，使它心跳加速，两翼无力，体重失去平衡，正如人们吃饭时，突然听见雷声，筷子落地的情况一样自然而平常。”

后人根据上面的故事，做成了“惊弓之鸟”这个成语，形容先前多次受过惊吓的人，忽然遇到同样可怕的事物，就吓得魂飞魄散，惊惶失措，不知如何去应付新的环境。

Source: http://chengyu.t086.com/gushi/330.html

Story in English

During the Warring States Period, there was a man called Geng Ying, whose archery skills were unparalleled in the world at that time.
One day, when he and the king of the state of Wei stood side by side, a group of swan geese flew over them. Geng Ying said confidently to the king: "I can shoot a bird just by the sound of bow."
The king was suspicious. Just at that time, a lone goose flew very flow and slowly over them. The cry was miserable. When Geng Ying saw this, he opened his bow without an arrow, struck the bow with a sound. The lone goose fell to the ground.
The king was amazed but didn't understand what was going on. Geng Ying explained: "That goose was flying low and slowly, because it had been hurt before, its cry was very sad, because the goose has been separated from the group. The body and soul were broken, which made its heart rate faster, its wings weaker and its weight out of balance. It is just as ordinary as when people are eating and suddenly hear a thunder, that their chopsticks fall to the ground."

Thus, according to the story, the idiom "A bird startled at the mere sound of a bow" denotes people who have in the past been shocked and frightened, and when something similar happens again, they are very spooked and do not know how to deal with this new environment.

Translation by Celine Li

Usage

金融市场对事情的反应往往出人意表，例如碰到一些似乎与市场毫不相干的事，它们可能会如惊弓之鸟，剧烈波动，相反面对一些市场敏感因素，却可能会不动如山。