The Collaborative International Dictionary of English v.0.48:

Fish-tail \Fish"-tail`\, a.
   Like the of a fish; acting, or producing something, like the
   tail of a fish.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

fishtail \fishtail\ v. t. (aeronautics)
   to slow down by moving the tail sideways, of an airplane.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

fishtail \fishtail\ v. i.
   to have the rear end skid from side to side, out of control:
   -- said of automobiles or other vehicles.
   [PJC]

   Fish-tail burner, a gas burner that gives a spreading flame
      shaped somewhat like the tail of a fish.

   Fish-tail propeller (Steamship), a propeller with a single
      blade that oscillates like the tail of a fish when
      swimming.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fishtail
    v 1: slow down by moving the tail sideways; "The airplane
         fishtailed on the runway"