1. [n]  parasitic flatworms having external suckers for attaching to a host
2. [n]  either of the two lobes of the tail of a cetacean
3. [n]  flat blade-like projection on the arm of an anchor
4. [n]  a barb on a harpoon or arrow
5. [n]  a stroke of luck

FLUKE is a 5 letter word that starts with F.

1. \Fluke\ (fl[=u]k), n. [Cf. AS. fl[=o]c a kind of flatfish,
   Icel. fl[=o]ki a kind of halibut.]
   1. (Zo["o]l.) The European flounder. See {Flounder}. [Written
      also {fleuk}, {flook}, and {flowk}].
   
   2. (Zo["o]l.) 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.
   
   
   
   

2. \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}.
   
   2. (Zo["o]l.) One of the lobes of a whale's tail, so called
      from the resemblance to the fluke of an anchor.
   
   3. An instrument for cleaning out a hole drilled in stone for
      blasting.
   
   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.
   
   

3. \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]