grim·ace 
(grĭmĭs, grĭ-mās)
(grĭmĭs, grĭ-mās)n.
A sharp contortion of the face expressive of pain, contempt, or disgust.
intr.v. grim·aced, grim·ac·ing, grim·ac·es
To make a sharp contortion of the face.
[French, from Old French grimache, alteration of grimuche, probably from Frankish *grīma, mask.]
grimac·er n.