Source : Webster's Revised Unabridged Dictionary (1913)

Fiddler \Fid"dler\, n. [AS. fi?elere.]
   1. One who plays on a fiddle or violin.

   2. (Zo["o]l.) A burrowing crab of the genus {Gelasimus}, of
      many species. The male has one claw very much enlarged,
      and often holds it in a position similar to that in which
      a musician holds a fiddle, hence the name; -- called also
      {calling crab}, {soldier crab}, and {fighting crab}.

   3. (Zo["o]l.) The common European sandpiper ({Tringoides
      hypoleucus}); -- so called because it continually
      oscillates its body.

   {Fiddler crab}. (Zo["o]l.) See {Fiddler}, n., 2.

Source : WordNet®

fiddler crab
     n : burrowing crab of American coastal regions having one claw
         much enlarged in the male