The Collaborative International Dictionary of English v.0.48:

Sardonic \Sar*don"ic\, a.
   Of, pertaining to, or resembling, a kind of linen made at
   Colchis.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sardonic \Sar*don"ic\, a. [F. sardonique, L. sardonius, Gr. ?,
   ?, perhaps fr. ? to grin like a dog, or from a certain plant
   of Sardinia, Gr. ?, which was said to screw up the face of
   the eater.]
   Forced; unnatural; insincere; hence, derisive, mocking,
   malignant, or bitterly sarcastic; -- applied only to a laugh,
   smile, or some facial semblance of gayety.
   [1913 Webster]

         Where strained, sardonic smiles are glozing still,
         And grief is forced to laugh against her will. --Sir H.
                                                  Wotton.
   [1913 Webster]

         The scornful, ferocious, sardonic grin of a bloody
         ruffian.                                 --Burke.
   [1913 Webster]

   Sardonic grin or Sardonic laugh, an old medical term for
      a spasmodic affection of the muscles of the face, giving
      it an appearance of laughter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sardonic
    adj 1: disdainfully or ironically humorous; scornful and
           mocking; "his rebellion is the bitter, sardonic laughter
           of all great satirists"- Frank Schoenberner; "a wry
           pleasure to be...reminded of all that one is missing"-
           Irwin Edman

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "sardonic":
   Rabelaisian, caustic, contemptuous, corrosive, cynical, derisive,
   derisory, disdainful, dry, ironic, ironical, jeering, mocking,
   sarcastic, satiric, saturnine, sneering, wry