The Collaborative International Dictionary of English v.0.48:

Rictus \Ric"tus\, n. [L., the aperture of the mouth.]
   The gape of the mouth, as of birds; -- often resricted to the
   corners of the mouth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rictus
    n 1: a gaping grimace