Source : Webster's Revised Unabridged Dictionary (1913)

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®

grinning
     See {grin}

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}]

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

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