poison-pen letter
A letter that contains malicious statements or accusations about the recipient or another party. Shortly after announcing his candidacy, he received an anonymous poison-pen letter attacking his platform.
poison-pen letter
A letter, usually anonymous, that makes malicious statements about the recipient or a third party. For example, She told the police about the poison-pen letters, but they said they couldn't pursue the matter . [Early 1900s]
a ˌpoison ˈpen letter
an unpleasant letter which is not signed and is intended to upset the person who receives it: Most politicians get poison pen letters, sometimes threatening their lives.