A malicious anonymous letter; an obscene crank letter (1929+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with 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