Rictus

rictus \ˈrik-təs\ noun: 1. the gape of a bird's mouth, 2 a. the mouth orifice, b. a gaping grin or grimace

Etymology: New Latin, from Latin, open mouth, from ringi to open the mouth

“His smile was the rictus of a beast.” — Salman Rushdie, The Enchantress of Florence