Wiktionary
alt. (context idiomatic English) A missive which is malicious, insulting, and/or defamatory toward a person, organization, or point of view, especially one which is unsigned. n. (context idiomatic English) A missive which is malicious, insulting, and/or defamatory toward a person, organization, or point of view, especially one which is unsigned.
Usage examples of "poison-pen letter".
I dug in my pockets and brought out the poison-pen letter that had scared Hopwood, plus the warm love-letter on sun-yellow paper that had lured him down the coast in the first place.
Her reaction to a poison-pen letter would be a shout of laughter and a clinical analysis of the mental state of the writer.
Some friend, she mused, to send her a poison-pen letter like that!
That's the why of your signature rule, to give you boys a chance to spot a poison-pen letter before it goes out.