Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.