The Collaborative International Dictionary of English v.0.48:

Cat o' nine tails \Cat" o' nine" tails`\, cat-o'-nine-tails
\cat"-o'-nine"-tails`\n.
   1. a whip used as an instrument of punishment consisting of
      nine pieces of knotted line or cord fastened to a handle;
      -- formerly used to flog offenders on the bare back; --
      called also the cat. It was used in the British Navy to
      maintain discipline on board sailing ships.

   Syn: cat.
        [WordNet 1.5 +PJC] Catopter