Rictus / RIK-tuhs / noun
1. The gape of the mouth, as of birds.
2. A gaping grin or grimace.
A rictus of cruel malignity lit up greyly their old bony faces.
-- James Joyce, A Portrait of the Artist as a Young Man
His belly swelled grotesquely, his hands curled, his cheeks puffed out, his mouth contorted in a rictus of pain and astonishment.
-- Tony Horwitz, Confederates in the Attic
Then, as the sympathy and praise engulfed him, Hector would invariably roll over onto his back, legs in the air, his mouth twisted into an otherworldly rictus.
-- Bruce McCall, "Writers Who Were Really Dogs", New York Times, June 5, 1994