Source : Webster's Revised Unabridged Dictionary (1913)

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