The Collaborative International Dictionary of English v.0.48:

Fluke \Fluke\ (fl[=u]k), n. [Cf. LG. flunk, flunka wing, the
   palm of an anchor; perh. akin to E. fly.]
   1. The part of an anchor which fastens in the ground; a
      flook. See Anchor.
      [1913 Webster]

   2. (Zool.) One of the lobes of a whale's tail, so called from
      the resemblance to the fluke of an anchor.
      [1913 Webster]

   3. An instrument for cleaning out a hole drilled in stone for
      blasting.
      [1913 Webster]

   4. An accidental and favorable stroke at billiards (called a
      scratch in the United States); hence, any accidental or
      unexpected advantage; as, he won by a fluke. [Cant, Eng.]
      --A. Trollope.
      [1913 Webster]

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]

The Collaborative International Dictionary of English v.0.48:

Fluke \Fluke\, v. t. & i. [imp. & p. p. Fluked; p. pr. & vb.
   n. Fluking.]
   To get or score by a fluke; as, to fluke a play in billiards.
   [Slang]
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

fluke
    n 1: a stroke of luck [syn: good luck, fluke, good
         fortune]
    2: a barb on a harpoon or arrow
    3: flat bladelike projection on the arm of an anchor [syn:
       fluke, flue]
    4: either of the two lobes of the tail of a cetacean
    5: parasitic flatworms having external suckers for attaching to
       a host [syn: fluke, trematode, trematode worm]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "fluke":
   Chilopoda, Chordata, Echiuroidea, Ectoprocta, Entoprocta,
   Monoplacophora, Nemertinea, Phoronidea, accident, adventure,
   blessing, break, casualty, chance hit, contingency, contingent,
   fortuity, freak accident, hap, happening, happenstance, hazard,
   long odds, long shot, lucky break, lucky shot, lucky strike,
   run of luck, serendipity, streak of luck, stroke of luck,
   windfall