poison pen

noun2

1. •
   A manner, means, or tone of expression had by a written work, characterized by spitefulness, defamation, and/or intimidation directed toward a person, organization, or point of view.

   • Peter Isaacson continued his attack on Hearst and the poison pen of his editors. — 1936
2. •
   [before a noun] Of, pertaining to, or possessing such a manner of expression.

   • In a fair political fight in which neither side wishes to take unfair advantage of an opponent, last-minute poison-pen circulars should be banned. — 1938