Source : Webster's Revised Unabridged Dictionary (1913)

Whip-poor-will \Whip"-poor-will`\, n. (Zo["o]l.)
   An American bird ({Antrostomus vociferus}) allied to the
   nighthawk and goatsucker; -- so called in imitation of the
   peculiar notes which it utters in the evening. [Written also
   {whippowil}.]