cris·sum 
(krĭsəm)
(krĭsəm)n. pl. cris·sa (krĭsə)
The feathers or area under the tail of a bird surrounding the cloacal opening.
[New Latin, from Latin crīsāre, crissāre, to move the buttocks during intercourse; see sker-2 in the Appendix of Indo-European roots.]
crissal (-əl) adj.