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