From The Collaborative International Dictionary of English v.0.48:

Uropygial \U`ro*pyg"i*al\, a. [See Uropygium.] (Anat.)
   Of or pertaining to the uropygium, or prominence at the base
   of the tail feathers, in birds.
   [1913 Webster]

   Uropygial gland, a peculiar sebaceous gland at the base of
      the tail feathers in most birds. It secretes an oily fluid
      which is spread over the feathers by preening.
      [1913 Webster]
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