sardonic

a. [ F. sardonique, L. sardonius, Gr. &unr_;, &unr_;, perhaps fr. &unr_; to grin like a dog, or from a certain plant of Sardinia, Gr. &unr_;, 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 ]