1
general::
see BELOW THE BELT., Not behave according to the rules or decency, unfairly, as in Bring ing 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.
American Heritage Idioms
2
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Phrase(s): hit someone below the belt
1. Lit. [for a boxer] to strike an opponent below the belt. (An unfair blow.) • The champ hit the contender below the belt and the crowd began to boo like fury. • Fred was hit below the belt and suffered considerably. 2. Fig. to deal someone an unfair blow. • That’s not fair! You told them I was the one who ordered the wrong-size carpet. That’s hitting me below the belt. • Todd hit below the belt when he said it was all her fault because she had become ill during the trip.
McGrawhill's American Idioms And Phrasal Verbs