The Collaborative International Dictionary of English v.0.48:

Grimace \Gri*mace"\ (gr[i^]m"[i^]s or gr[i^]*m[=a]s"), n. [F.,
   prob. of Teutonic origin; cf. AS. gr[imac]ma mask, specter,
   Icel. gr[imac]ma mask, hood, perh. akin to E. grin.]
   A distortion of the countenance, whether habitual, from
   affectation, or momentary and occasional, to express some
   feeling, as contempt, disapprobation, complacency, etc.; a
   smirk; a made-up face.
   [1913 Webster]

         Moving his face into such a hideous grimace, that every
         feature of it appeared under a different distortion.
                                                  --Addison.
   [1913 Webster]

   Note: "Half the French words used affectedly by Melantha in
         Dryden's `Marriage a-la-Mode,[rsquo] as innovations in
         our language, are now in common use: chagrin,
         double-entendre, ['e]claircissement, embarras,
         ['e]quivoque, foible, grimace, na["i]vete, ridicule.
         All these words, which she learns by heart to use
         occasionally, are now in common use." --I. Disraeli.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grimace \Gri*mace"\, v. i.
   To make grimaces; to distort one's face; to make faces. --H.
   Martineau.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grimace
    n 1: a contorted facial expression; "she made a grimace at the
         prospect" [syn: grimace, face]
    v 1: contort the face to indicate a certain mental or emotional
         state; "He grimaced when he saw the amount of homework he
         had to do" [syn: grimace, make a face, pull a face]