The Collaborative International Dictionary of English v.0.48:

outrigger \out"rig`ger\, n.
   1. Any spar or projecting timber run out for temporary use,
      as from a ship's mast, to hold a rope or a sail extended,
      or from a building, to support hoisting teckle.
      [1913 Webster]

   2. (Naut.)
      (a) A projecting support for a rowlock, extended from the
          side of a boat.
      (b) A boat thus equipped.
      (c) A projecting contrivance at the side of a boat to
          prevent upsetting, as projecting spars with a log at
          the end; also used attributively, as an outrigger
          canoe.
          [1913 Webster +PJC]

   3. (Aeronautics) A projecting frame used to support the
      elevator or tail planes, etc.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

outrigger
    n 1: a stabilizer for a canoe; spars attach to a shaped log or
         float parallel to the hull