The unjust damaging of the good name of another by imputing to him a crime or fault of which he is not guilty. The sin thus committed is in a general sense mortal, just as is detraction. It is hardly necessary, however, to observe that as in other breaches of the law the sin may be venial, either because of the trivial character of the subject-matter involved or because of insufficient deliberation in the making of the accusation. Objectively, a calumny is a mortal sin when it is calculated to do serious harm to the person so traduced. Just as in the instance of wrongful damage to person or estate, so the calumniator is bound to adequate reparation for the injury perpetrated by the blackening of another's good name.
It's a sin against the Eighth Commandment.