Which term describes an abusive attack on a person's character or reputation?

The term that refers to an abusive attack on a person's character or reputation is "calumny." Calumny specifically denotes false and malicious statements that are made to tarnish someone's reputation, often intentionally misleading others about the individual in question. This aligns closely with the concept of character assassination, where the intent is to harm someone's public image and standing.

In contrast, "defalcate" pertains to the act of misappropriating funds or embezzlement, which does not relate to character attacks. "Cavil" describes raising trivial objections or making petty criticisms, which lacks the malicious intent associated with calumny. Lastly, "countenance" refers to a person's facial expression or support for something, and does not have any connection to abusive attacks on reputation. Therefore, calumny is the most fitting term to describe this type of defamatory action.