The Collaborative International Dictionary of English v.0.48:

Grin \Grin\, v. i. [imp. & p. p. Grinned; p. pr. & vb. n.
   Grinning.] [OE. grinnen, grennen, AS. grennian, Sw. grina;
   akin to D. grijnen, G. greinen, OHG. grinan, Dan. grine.
   [root]35. Cf. Groan.]
   1. To show the teeth, as a dog; to snarl.
      [1913 Webster]

   2. To set the teeth together and open the lips, or to open
      the mouth and withdraw the lips from the teeth, so as to
      show them, as in laughter, scorn, or pain.
      [1913 Webster]

            The pangs of death do make him grin.  --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grinning
    n 1: a facial expression characterized by turning up the corners
         of the mouth; usually shows pleasure or amusement [syn:
         smile, smiling, grin, grinning]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "grinning":
   banter, bantering, beam, booing, bright smile, broad grin,
   catcalling, chaffing, derision, derisive, derisory,
   ear-to-ear grin, fleering, flippancy, flippant, fooling,
   gleaming smile, glowing smile, grin, hissing, hooting,
   idiotic grin, jeering, joshing, kidding, leering, levity, mockery,
   mocking, panning, quizzical, ragging, railing, raillery, rallying,
   razzing, ridicule, ridiculing, roasting, sardonic grin, scoffing,
   simper, smart, smart-aleckiness, smart-alecky, smart-ass,
   smartness, smile, smiling, smirk, smirking, sneering, snickering,
   sniggering, snorting, stupid grin, taunting, teasing,
   toothful grin, twitting