The Collaborative International Dictionary of English v.0.48:

Fluke \Fluke\ (fl[=u]k or fl[=oo]k), n. [Cf. AS. fl[=o]c a kind
   of flatfish, Icel. fl[=o]ki a kind of halibut.]
   1. (Zool.) The European flounder. See Flounder. [Written
      also fleuk, flook, and flowk.]
      [1913 Webster]

   2. (Zool.) Any American flounder of the genus Paralichthys,
      especially Paralicthys dentatus, found in the Atlantic
      Ocean and in adjacent bays. --RHUD
      [PJC]

   3. (Zool.) A parasitic trematode worm of several species,
      having a flat, lanceolate body and two suckers. Two
      species (Fasciola hepatica and Distoma lanceolatum)
      are found in the livers of sheep, and produce the disease
      called rot.
      [1913 Webster]