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1. adjective (hyphenated and used before a noun) Spiteful, malicious, or defamatory in composition, as of a letter or other written work, generally sent anonymously and with the aim of damaging or destroying a person's, group's, or organization's reputation or happiness. After agreeing to defend the alleged mass-murderer, the public defendant began receiving a large number of poison-pen letters from around the city. We've had enough trouble getting this restaurant up and running without having a poison-pen review like that in the papers!
2. adjective (hyphenated and used before a noun) Characterized by, typified by, or inclined toward the writing and sending of such letters or pieces of writing. The outspoken feminist figure has been the target of a hateful poison-pen campaign by online misogynists who have hidden behind the shield of anonymity to conduct their attacks.
3. noun A disposition, attitude, inclination, or ability typified by the writing and sending such letters or pieces of writing. Every year or so, there is a new piece of diatribe and vitriol from the poison pen of the infamous white supremacist. The poison pen of the oft-feared but well-respected critic has been enough to cause the downfall of more than one virtuoso's career.