The Collaborative International Dictionary of English v.0.48:

Spencer \Spen"cer\, n. (Naut.)
   A fore-and-aft sail, abaft the foremast or the mainmast,
   hoisted upon a small supplementary mast and set with a gaff
   and no boom; a trysail carried at the foremast or mainmast;
   -- named after its inventor, Knight Spencer, of England
   [1802].
   [1913 Webster]

   Spencer mast, a small mast just abaft the foremast or
      mainmast, for hoisting the spencer. --R. H. Dana, Jr.
      [1913 Webster]