The Collaborative International Dictionary of English v.0.48:

Uropygium \U`ro*pyg"i*um\, n. [NL., fr. Gr. ?, (corrupted form)
   ?; ? the end of the os sacrum + ? rump.] (Anat.)
   The prominence at the posterior extremity of a bird's body,
   which supports the feathers of the tail; the rump; --
   sometimes called pope's nose.
   [1913 Webster]

WordNet (r) 3.0 (2006):

uropygium
    n 1: posterior part of a bird's body from which the tail
         feathers grow