Source : Webster's Revised Unabridged Dictionary (1913)

Bitter \Bit"ter\, n. [See {Bitts}.] (Naut.)
   AA turn of the cable which is round the bitts.

   {Bitter end}, that part of a cable which is abaft the bitts,
      and so within board, when the ship rides at anchor.

Source : WordNet®

bitter end
     n 1: the final extremity (however unpleasant it may be); "he was
          determined to fight to the bitter end"
     2: (nautical) the inboard end of a line or cable especially the
        end that is wound around a bitt