IDIOMS

poison pen

(attributive, sometimes hyphenated) Of, pertaining to, or possessing such a manner of expression.

Example: 1936, "Maine Politics Afield," Lewiston Evening Journal (USA), 31 Aug. (retrieved 27 Aug. 2010):
  Peter Isaacson continued his attack on Hearst and the poison pen of his editors.
2003, Marc Santora, "Rowland Vows to Stay On as Governor," New York Times, 17 Dec. (retrieved 27 Aug. 2010):
  After Mr. Rowland spoke, his wife, Patricia, delivered a spoof on the poem "Twas the Night Before Christmas," that was a stinging sing-song rebuke of the news organizations that had reported on her husband's shortcomings and the widening federal investigation into his administration. Referring to the news media, she said: "They used to be good girls and boys, Santa said. But the poison pen's power has gone to their head."
2004, Matt Wells, "Lynda Lee-Potter, doyenne of columnists, dies," guardian.co.uk, 21 Oct. (retrieved 27 Aug. 2010):
  The centrepiece of her work was a weekly column that could brim with venom . . . . Cherie Blair, Geri Halliwell, Sarah Ferguson, Victoria Beckham and Princess Diana all found themselves the subjects of Lee-Potter's poison pen.
1938, "Editorial: Watch Out For The Poison Pen," The Afro-American, 5 Nov., p. 4 (retrieved 27 Aug. 2010):
  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.
1982, Anthony Lewis, "The poisoning of the well," St. Petersburg Times, 25 May, p. 10A (retrieved 27 Aug. 2010):
  This collection of garbage and guilt-by-association was slipped into the files of the White House personnel office last fall. . . . Who was responsible for the poison-pen attack? No one signed it, and no one has taken responsibility.
2004, Anthony McCarron, "Baseball in the Bronx playing like New York's greatest theater," New York Daily News, 24 Aug. (retrieved 27 Aug. 2010):
  He's been harsh, too, reacting with the swift kick of a poison-pen reviewer when he doesn't like what he sees on his stage.