Source : Webster's Revised Unabridged Dictionary (1913)

Grin \Grin\, v. t.
   To express by grinning.

         Grinned horrible a ghastly smile.        --Milton.

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.

         He showed twenty teeth at a grin.        --Addison.

Grin \Grin\ (gr[i^]n), n. [AS. grin.]
   A snare; a gin. [Obs.]

         Like a bird that hasteth to his grin.    --Remedy of
                                                  Love.

Grin \Grin\, v. i. [imp. & p. p. {Grinned} (gr[i^]nd); 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.

   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.

            The pangs of death do make him grin.  --Shak.

Source : WordNet®

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

grin
     v : to draw back the lips and reveal the teeth, in a smile,
         grimace, or snarl
     [also: {grinning}, {grinned}]