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hit below the belt (in an unfair or cowardly way, not following the rules ) — нанести удар ниже пояса

I think that he was hitting below the belt when he began to criticize me after I told him the truth.

It was a case of hitting someone below the belt when the girl stole her best friend's boyfriend from her.

see also
[stab sb in the back]
[foul play]