a letter written in malice, usually anonymously, and intended to abuse, frighten, or insult the recipient
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.