Injuring another person’s good name by lying. It is doubly sinful, in unjustly depriving another of his good name and in telling an untruth. Since calumny violates justice, it involves the duty of making reparation for the foreseen injury inflicted. Hence the calumniator must try, not only to repair the harm done to another’s good name, but also to make up for any foreseen temporal loss that resulted from the calumny, for example, loss of employment or customers. (Etym. Latin calumnia, a false accusation, malicious charge; from calvi, to deceive.)
Modern Catholic Dictionary
by Fr. John A. Hardon, S.J.