What is "poison pen letter"

n. (alternative form of poison-pen letter English)

A poison pen letter is a letter or note containing unpleasant, abusive or malicious statements or accusations about the recipient or a third party. It is usually sent anonymously. Poison pen letters are usually composed and sent to upset the recipient. They differ from blackmail, which is intended to obtain something, in that they are purely malicious.

In the United Kingdom, Section 1 of the Malicious Communications Act 1988 covers most cases of poison pen letters.