Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† orthŏpȳgĭum, ĭi, n., = ὀρθοπύγιον, the rump and tail feathers of birds, Mart. 3, 93, 12 (al. orrhopygium = ὀρροπύγιον, of the same meaning).