hit (one) below the belt
To unfairly target one's weakness or vulnerability. The phrase refers to boxing, in which hitting an opponent below the waist is prohibited. I know she really wants the promotion, but she really hit me below the belt by telling the boss about my personal problems.
hit someone below the belt
behave deviously towards someone, especially so as to gain an unfair advantage. In boxing, delivering a blow below an opponent's waistline is against the rules.
hit someone below the belt
tv. to deal with someone unfairly; to exploit someone’s vulnerabilities unfairly. (From boxing.) You were hitting Tom below the belt when you said that.