Plural of crissus; the area around the cloaca (waste opening) in birds and other animals.
Latin plural of crissus, from Latin crista meaning crest. In anatomical terminology, it refers to the region of an animal's posterior end, particularly in ornithology (bird study).
This is highly technical ornithological vocabulary—'crissa' is what bird scientists call the undertail area, and knowing it immediately identifies someone as speaking the specialized language of bird anatomy.
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