An unfair and undeserved action done to others solely for one's own benefit; a cheap shot

It was really a low blow telling Paola that the guy of her dreams didn't even know she existed

È stato davvero un colpo basso dire a Paola che il ragazzo dei suoi sogni non la vede nemmeno

That was surely a low blow on your part: telling my guests that the wonderful dinner I was so proud of, was secretly prepared by a professional chef

Potevi evitarmi questa carognata: dire ai miei ospiti che la splendida cena di cui andavo tanto fiera era, in realtà, opera di un cuoco professionista

In boxing it is illegal to hit the adversary below the belt, which is literaclly a low blow