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]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "grimace":
   ache, agonize, ail, anguish, blanch, blench, bob, bobble, bounce,
   bump, contort, declaim, deform, didder, disapprove of, distort,
   dither, face, falter, feel pain, feel the pangs, frown, gloom,
   glower, ham, ham it up, have a misery, hurt, jar, jerk, jig,
   jigget, jiggle, jog, joggle, jolt, jostle, knit the brow,
   long face, look askance at, look black, look sullen, lower,
   make a face, make a lip, make a moue, make a mouth, misshape, mop,
   mop and mow, moue, mouth, mouthing, mow, mug, out-herod Herod,
   overact, overdramatize, pound, pout, pull a face, quake, quaver,
   quiver, rant, recoil, revolt at, rictus, roar, scowl, shake,
   shiver, shock, shoot, show distaste for, shrink, shrink from,
   shudder, shudder at, smart, snarl, spout, suffer, thrill, throb,
   throw away, tic, tingle, tremble, tremor, twinge, twitch, twitter,
   underact, wince, wobble, writhe, wry face, wry mouth