hit someone where it hurts idiom
刺到（某人）的痛处，碰到（某人）的敏感之处
to do or say something to someone that will upset them as much as possible
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。