The OED defines it as:

  1. False and malicious misrepresentation of the words or actions of others, calculated to injure their reputation; libellous detraction, slander.
  2. A false charge or imputation, intended to damage another’s reputation; a slanderous report.

This reveals a range of meaning, and the word is not used in English law. But in Roman law, the word had a specific meaning: calumnia denoted a malicious prosecution knowingly proceeded with. A reus who was acquitted might consider an action against his prosecutor for calumnia, and the penalty for that might be banishment, or—as appears from Cicero’s speech Pro Roscio Amerino—to have the letter K for Kalumniator branded on his forehead, that being the ancient spelling of the word.

The offence of calumnia had been defined by the Lex Remmia, but little detail is available. What is interesting is that Cicero’s speech in defence of Roscius of Ameria, which was his first criminal case, established his reputation for both oratory and courage. Roscius was in court accused of having murdered his father, and was being prosecuted in a conspiracy by the very people who had actually committed the murder. It was calumnia at its most outrageous: the conspiracy was instigated by Chrysogonus, freedman of the dictator Sulla; and it seems to have been general knowledge that the motive for the murder was to obtain the dead man’s property: for he was a rich landowner.

In that speech (Ch.84) occurs the memorable question cui bono? — ‘Who stood to gain?’ But those two words are not Cicero’s: he was quoting them from Lucius Cassius Longinus Ravilla, consul in 127BC, who had used that principle in judging criminal cases. Cicero uses the words again in Pro Milone Ch.12.

In our legal system, the malicious prosecution of Roscius would not have got as far as the courts, for the prosecutor Erucius, who was a puppet of Chrysogonus, actually had no evidence to present.

Of interest:*/Calumnia.html