Wikipedia
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.
Usage examples of "risus sardonicus".
Apart from the inventor of the double-bottomed defecator, whose history he had heard seven times, they had nothing to say, and they said it for what seemed hours, while his smile grew more fixed and rigid until at last it came to resemble the risus sardonicus.
Coupled with this distortion of the face, this Hippocratic smile, or 'risus sardonicus,' as the old writers called it, what conclusion would it suggest to your mind?