Risus sardonicus

Risus sardonicus or rictus grin is a highly characteristic, abnormal, sustained spasm of the facial muscles that appears to produce grinning.

The name of the condition, which has its roots in the Mediterranean island of Sardinia, derives from the appearance of raised eyebrows and an open "grin" – which can appear sardonic or malevolent to the lay observer – displayed by those suffering from these muscle spasms.