Meaning- cruel, unfair,hurtful
Usage – Saying he wasn’t very good at English just because he made one mistake was really heating below the belt.
Origin- The sport of boxing can be dangerous. Prizefighters can get seriously injured if the official rules of the game are not followed. Most of the rules in effect today are based on those proposed in 1867 by the Marquis of Queensberry , a British nobleman.He said punching below the belt not allowed. Today boxers wear trunks with waistbands , not belts, but the rule is still followed.