打落水狗 to beat a dog when it's down
Explanation
比喻乘人之危，继续打击已经失败的敌人或对手。
To continue to attack someone who has already been defeated; to take advantage of someone's weakness and exploit them ruthlessly.
Origin Story
话说三国时期，蜀汉名将赵云在长坂坡大战中，以一己之力，七进七出，救出阿斗。曹操率领大军追杀，赵云英勇无比，杀得曹军人仰马翻。这时，赵云已经精疲力尽，曹操见此情景，本想下令追杀赵云，但是考虑到赵云武艺高强，又怕折损兵力，曹操便下令停止追击，赵云这才得以幸免。这个故事，并非字面意义上的“打落水狗”，而是说明了一个道理：即使敌方已经处于弱势，也不应该掉以轻心，要懂得见好就收。
During the Three Kingdoms period in China, the famous Shu Han general Zhao Yun, in the great battle of Changbanpo, single-handedly rescued A Dou seven times. Cao Cao led a large army to pursue and kill Zhao Yun, but Zhao Yun was incredibly brave and killed the Cao army. At this point, Zhao Yun was exhausted, and Cao Cao, seeing this, wanted to order the pursuit and killing of Zhao Yun, but considering Zhao Yun's martial arts and fearing losses, Cao Cao ordered the pursuit to stop. Zhao Yun was thus able to escape. This story is not a literal interpretation of 'kicking a dog when it is down', but illustrates an important point: even if the opposing side is already in a weak position, one should not be careless and should know when to stop.
Usage
通常作谓语、宾语；比喻乘人之危，继续打击已经失败的敌人或对手。
Usually used as a predicate or object; to continue to attack someone who has already been defeated; to take advantage of someone's weakness and exploit them ruthlessly.
Examples
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面对失败的敌人，我们不能掉以轻心，要乘胜追击，打落水狗。
miànduì shībài de dírén, wǒmen bù néng diào yǐ qīngxīn, yào chéngshèng zhuījí, dǎ luò shuǐ gǒu
We can't take it easy on a defeated enemy. Instead, we should take advantage of the situation and strike while the iron is hot.
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这次竞争对手已经溃不成军，我们应该抓住机会，打落水狗，彻底击垮他们。
zhè cì jìngzhēng duìshǒu yǐjīng kuì bù chéng jūn, wǒmen yīnggāi zhuā zhù jīhuì, dǎ luò shuǐ gǒu, chèdǐ jī kuà tāmen
Our competitor is already in a state of collapse, we should seize the opportunity to deliver a final blow and crush them completely.