To make things worse for someone who is already in a difficult situation; to hit a person when they are down; to take advantage of someone's misfortune to harm them further.
小明考试失利已经很沮丧了，没想到他的朋友小华还在一旁冷嘲热讽，真是投阱下石。
Xiao Ming was already very upset about failing his exam, but unexpectedly, his friend Xiao Hua was still making sarcastic remarks, truly hitting him when he was down.
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