The Collaborative International Dictionary
Crissum
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.
Wiktionary
crissum
n. The area of feathers on a bird that surround its cloaca.