In ancient times there was a county magistrate who took bribes and practised graft. One day, somebody sent him a petition accusing his secretary of practising graft and taking bribes. The magistrate trembles when he read the petition. He wrote on it:"You have beaten the grass and frightened a snake."
This idiom refers to alerting the target of one's scheme by being incautious.
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!
后来，大家就根据王鲁所写的八个字“汝虽打草，吾已蛇惊”，引伸为“打草惊蛇”这句成语。
