What does "sardonic" mean?

Sardonic: 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. Wotton. The scornful, ferocious, sardonic grin of a bloody ruffian. Burke. Sardonic grin or laugh, an old medical term for a spasmodic affection of the muscles of the face, giving it an appearance of laughter.