The Collaborative International Dictionary of English v.0.48:

Rook \Rook\, n. [AS. hr[=o]c; akin to OHG. hruoh, ruoh, ruoho,
   Icel. hr[=o]kr, Sw. roka, Dan. raage; cf. Goth. hrukjan to
   crow.]
   1. (Zool.) A European bird (Corvus frugilegus) resembling
      the crow, but smaller. It is black, with purple and violet
      reflections. The base of the beak and the region around it
      are covered with a rough, scabrous skin, which in old
      birds is whitish. It is gregarious in its habits. The name
      is also applied to related Asiatic species.
      [1913 Webster]

            The rook . . . should be treated as the farmer's
            friend.                               --Pennant.
      [1913 Webster]

   2. A trickish, rapacious fellow; a cheat; a sharper.
      --Wycherley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Corvus frugilegus
    n 1: common gregarious Old World bird about the size and color
         of the American crow [syn: rook, Corvus frugilegus]