The Collaborative International Dictionary of English v.0.48:

Grin \Grin\, n. [AS. grin.]
   A snare; a gin. [Obs.]
   [1913 Webster]

         Like a bird that hasteth to his grin.    --Remedy of
                                                  Love.
   [1913 Webster]

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]

The Collaborative International Dictionary of English v.0.48:

Grin \Grin\, v. t.
   To express by grinning.
   [1913 Webster]

         Grinned horrible a ghastly smile.        --Milton.

The Collaborative International Dictionary of English v.0.48:

Grin \Grin\, n.
   The act of closing the teeth and showing them, or of
   withdrawing the lips and showing the teeth; a hard, forced,
   or sneering smile. --I.Watts.
   [1913 Webster]

         He showed twenty teeth at a grin.        --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grin
    n 1: a facial expression characterized by turning up the corners
         of the mouth; usually shows pleasure or amusement [syn:
         smile, smiling, grin, grinning]
    v 1: to draw back the lips and reveal the teeth, in a smile,
         grimace, or snarl

Moby Thesaurus II by Grady Ward, 1.0:

17 Moby Thesaurus words for "grin":
   beam, bright smile, broad grin, crack a smile, ear-to-ear grin,
   gleaming smile, glowing smile, grinning, idiotic grin,
   sardonic grin, simper, smile, smile brightly, smiling, smirk,
   stupid grin, toothful grin