below the belt
Definition from Wiktionary, the free dictionary
Prepositional phrase 
- (boxing) Of a punch, landing illegally, below the opponent's waist.
- (by extension, idiomatic) Unfair; dirty; not according to the generally accepted rules.
- Commenting on his bachelorhood in a debate on marital ethics was a bit below the belt, don't you think?
Derived terms 
- hit below the belt, most common verb form
(boxing) of a punch that lands below the opponent's waist
unfair; not according to the rules