the.com/below the belt

means An unfair attack that targets something too personal or vulnerable to be a legitimate part of the fight.

from Straight out of the boxing ring: rules of the sport mark the belt line as the boundary, and punches landed below it — toward the groin — are fouls, however easy they are to throw. By the 1800s the phrase had already climbed out of the ring and into everyday argument, describing any jab, verbal or otherwise, aimed at a weak spot that fair fighting is supposed to leave alone.