/ˈpɔɪzən pɛn ˈlɛtər/ – Phrase
Definition: Thư độc hại hoặc thư đe dọa.
A more thorough explanation: an anonymous letter that makes abusive or defamatory statements (=untrue statements designed to cause harm) against someone
Example: The couple were sent a poison-pen letter about their “bad taste”‘ garden furniture.