hit sb where it hurts

To attack one in the area that is most vulnerable or that will result in the most harm. Usually does not refer to physical violence.

攻击一个最脆弱的区域而导致最大的伤害。通常不是指身体暴力。

刺到(某人)的痛处，碰到(某人)的敏感之处

He's always worrying about his weight, so if you want to hit him where it hurts, tell him he's looking a bit fat.

他一直在担心自己的体重，如果你想刺到他的痛处，就说他有点胖。