落井投石 throw stones into the well
Explanation
比喻乘人之危加以打击、陷害。看见别人遇到困难或不幸，不仅不帮助，反而落井下石，进行打击陷害。
A metaphor for taking advantage of someone's misfortune to attack and harm them. When someone encounters difficulties or misfortune, not only do you not help them, but you also throw stones into the well and attack them.
Origin Story
从前，村里有个爱占便宜的李老汉。一天，他看见张老汉不小心掉进了一口枯井里。李老汉不仅没帮忙，反而从地上捡起石头，往井里扔。张老汉在井底痛哭流涕，大声喊叫，李老汉不但没有丝毫同情，反而扔得更起劲了。最终，张老汉在井底冻饿而死。后来，李老汉因为自己的行为遭到村民的谴责，被村里的人孤立，郁郁寡欢地度过了余生。这个故事警示人们要乐于助人，不能见死不救，更不能落井投石，陷害他人。
Once upon a time, there was a villager named Old Man Li who loved to take advantage of others. One day, he saw Old Man Zhang accidentally fall into a dry well. Old Man Li not only didn't help, but also picked up stones from the ground and threw them into the well. Old Man Zhang cried bitterly at the bottom of the well, shouting loudly, but Old Man Li showed no sympathy, instead throwing even more stones. Eventually, Old Man Zhang died of cold and hunger at the bottom of the well. Later, Old Man Li was condemned by the villagers for his actions and isolated by them. He spent the rest of his life in sadness and despair. This story warns people to be willing to help others, not to stand by and watch others die, and not to throw stones into the well to harm others.
Usage
常用作谓语、定语；形容乘人之危，落井下石的行为。
Often used as a predicate or attributive; describes taking advantage of others' misfortunes and intentionally causing harm.
Examples
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他落井下石的行为让人愤慨。
tā luò jǐng xià shí de xíngwéi ràng rén fènkǎi
His act of throwing stones into the well is infuriating.
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面对竞争对手的失败，他不落井投石，反而伸出援手。
miàn duì jìngzhēng duìshǒu de shībài, tā bù luò jǐng tóu shí, fǎn'ér shēn chū yuánshǒu
Faced with the failure of his competitors, he didn't throw stones into the well, but instead offered a helping hand.