Meanings of poison pen letter in English

Oh and thanks Alfred, your comment reminded me that I need to write a poison pen letter.

Germany's Bundesbank has sent a poison pen letter to Berlin warning that only drastic measures can save Europe's largest economy from a crisis.

Count Julian was once described as a " poison pen letter to Spanish society", because it sent up the Catholic cult of virginity.

Enid, it appears, was a hard case, a ruthless scrooge and cheat who wrote poison pen letters between penning children's books and playing strip tennis.