defamer

This term carries a strong legal and moral weight, specifically referring to the actor in a case of defamation. It is far more formal and accusatory than words like gossip or liar, as it implies a deliberate, malicious intent to cause tangible harm to someone's social or professional standing.

In contemporary usage, the word is relatively rare in casual conversation, appearing most frequently in legal contexts, formal complaints, or high-stakes political rhetoric. It describes the source of the slander or libel rather than the act itself.