below the belt
below the belt {adv. phr.}
1. In the stomach; lower than is legal in boxing.
He struck the other boy below the belt.
2. {informal}
In an unfair or cowardly way; against the rules of sportsmanship or justice; unsportingly; wrongly.
It was hitting below the belt for Mr. Jones's rival to tell people about a crime that Mr. Jones committed when he was a young boy.
Pete told the students to vote against Harry because Harry was in a wheelchair and couldn't be a good class president, but the students thought Pete was hitting below the belt.
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Source: A Dictionary of American Idioms