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
Forced; unnatural; insincere; hence, derisive, mocking,
malignant, or bitterly sarcastic; -- applied only to a laugh,
smile, or some facial semblance of gayety.
Where strained, sardonic smiles are glozing still,
And grief is forced to laugh against her will. --Sir H.
The scornful, ferocious, sardonic grin of a bloody
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.