Hit below the belt. To say something that is often too personal, usually irrelevant, and always unfair. “To remind reformed alcoholics of their drinking problem is to hit below the belt.” The expression comes from boxing, in which it is illegal to hit an opponent below the belt.
Here on the channel you can find not only grammar materials, but also voice messages, quotations, idioms, phrasal verbs, tests, definitions and other things devoted to improvement of your english knowledge. For questions - @oleole4ka