Poison-Pen Letter Meaning

(idiomatic) A missive which is malicious, insulting, and/or defamatory toward a person, organization, or point of view, especially one which is unsigned.

Example: 1915, "Poison Pen Writer Released on Bail," New York Times, 18 Dec., p. 7 (retrieved 25 Aug. 2010):
  Miss Theresa Samuels . . . is accused by the Post Office authorities of sending scurrilous letters to young women whose engagements to marry had been publicly announced. . . . Those who received the poison pen letters were not willing to court the notoriety that would follow the prosecution of the case.
2001, Nigel Bunyan, "Twelve-year reign of poison pen villager," telegraph.co.uk, 1 Aug. (retrieved 25 Aug. 2010):
  A retired academic was the author of a poison-pen letter campaign that brought 12 years of fear to a North Yorkshire village, a court was told yesterday. Dr James Forster . . . was alleged to have branded one villager a prostitute and sent the 13-year-old daughter of the parish clerk a copy of a pornographic magazine.