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 wheel chair and couldn't be a good class president, but the students thought Pete was hitting below the belt.