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English Translation

lit.: to beat the grass and frighten away the snake
fig.: to act rashly and inadvertently alert an enemy

Breaking it down

1. 打: to beat
2. 草: grass
3. 惊: surprise, shock, alarm
4. 蛇: snake

Source: Google Image Search

Story in Chinese

唐朝的时候，有一个名叫王鲁的人，他在衙门做官的时候，常常接受贿赂、不遵守法规。有一天，有人递了一张状纸到衙门，控告王鲁的部下违法、接受贿赂。
王鲁一看，状纸上所写的各种罪状，和他自己平日的违法行为一模一样。王鲁一边看着状纸，一边发着抖：“这...这不是在说我吗？”
王鲁愈看愈害怕，都忘状纸要怎么批，居然在状纸上写下了八个大字：“汝虽打草，吾已蛇惊。”意思就是说你这样做，目的是为了打地上的草，但我就像是躲在草里面的蛇一样，可是被大大的吓了一跳了！

Source: http://chengyu.t086.com/gushi/44.html

Story in English

During the Tang Dynasty, there was a man named Wang Lu. As an official, he often accepted bribes and did not abide by the regulations.
One day, someone wrote a letter accusing Wand Lu's subordinates of breaking the law and accepting bribes. Wang Lu read the letter and saw that the various crimes written on it were exactly the same as his usual illegal activities. He trembled while looking at the paper: "This... Is this talking about me?"
He was so scared and frightened that he even forgot how to criticize the paper. He even wrote unto the paper: "You beat the grass, and I am already frightened like a snake." The meaning of this is that you writing this letter, with the goal of beating the grass, has the effect that I - like the snake hidden in the grass - is really shocked and taken aback!

Translation by Celine Li

Usage

这次行动千万要保密,不能打草惊蛇。