hit below the belt
To unfairly target another person's weakness or vulnerability. The phrase refers to boxing, in which hitting an opponent below the waist is unacceptable. I know she really wants the promotion, but she hit below the belt when she told the boss about my personal problems.
below the belt, hit
Not behave according to the rules or decency, unfairly, as in Bringing up my mother's faults-that's really hitting below the belt. The term comes from boxing, where according to the Marquis of Queensberry Rules (1865) a fighter may punch his opponent only in the upper body or head. For a synonym, see low blow; also see under one's belt.