Risus sardonicus

Risus sardonicus is a highly characteristic, abnormal, sustained spasm of the facial muscles that is most often observed as a symptom of tetanus. The name of the condition derives from the appearance of raised eyebrows and an evil, open grin that it gives to its victim. Also poisoning with strychnine may result in a risus sardonicus.

In popular culture
The condition was the inspiration behind the 1981 track Sardonicus by UB40, on their album, Present Arms. The song has strong political linkages with false smiles.