The Collaborative International Dictionary of English v.0.48:

Crissum \Cris"sum\ (kr?s"s?m),, n.; pl. Crissa (-s?). [NL.;
   cf. L. crisso to move the haunches.] (Zool.)
   That part of a bird, or the feathers, surrounding the cloacal
   opening; the under tail coverts.
   [1913 Webster]