The Collaborative International Dictionary of English v.0.48:

Spanker \Spank"er\ (sp[a^][ng]k"[~e]r), n.
   1. One who spanks, or anything used as an instrument for
      spanking.
      [1913 Webster]

   2. (Naut.) The after sail of a ship or bark, being a
      fore-and-aft sail attached to a boom and gaff; --
      sometimes called driver. See Illust. under Sail.
      --Totten.
      [1913 Webster]

   3. One who takes long, quick strides in walking; also, a fast
      horse. [Colloq.]
      [1913 Webster]

   4. Something very large, or larger than common; a whopper, as
      a stout or tall person. [Colloq.]
      [1913 Webster]

   Spanker boom (Naut.), a boom to which a spanker sail is
      attached. See Illust. of Ship.
      [1913 Webster]