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Adjective: viperous  vI-pu-rus
  1. Marked by deep ill will; deliberately harmful
    "viperous gossip"; "took viperous pleasure in...watching me wince";
    - poisonous, venomous, vicious, malicious
  2. Of or pertaining to a viper or vipers; resembling a viper
    - viperine

See also: beady-eyed, bitchy [informal], cattish, catty, despiteful, leering, malevolent, spiteful, venomed, vindictive, vixenish