Fig.: to act unfairly in any contest solely for one's own interest. To violate rules or accepted standards of conduct

Saying that I shouldn't represent our club just because I'm a little overweight is really hitting below the belt

Dire che non dovrei rappresentare il nostro club solo perché sono leggermente in sovrappeso è tirare un colpo basso

The journalist was fired for hitting below the belt at the editor-in-chief by trying to tarnish his name with false accusations

Il giornalista è stato licenziato per aver tirato un colpo basso al capo editore cercando di infangarne il nome con false accuse

One of the rules of the Marquis of Queensberry for boxing, published in England in 1867, forbid boxers to hit their opponents below the waist belt. Today this expression can be used in any contest