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Phrase(s): touch a sore spot [and] touch a sore point
Fig. to refer to a sensitive matter that will upset someone. (Fig. on the notion of touching an injury and causing pain.) • I seem to have touched a sore spot. I’m sorry. I didn’t mean to upset you. • When you talk to him, avoid talking about money. It’s best not to touch a sore point if possible.
McGrawhill's American Idioms And Phrasal Verbs