The Collaborative International Dictionary of English v.0.48:

Bitter \Bit"ter\, n. [See Bitts.] (Naut.)
   AA turn of the cable which is round the bitts.
   [1913 Webster]

   Bitter end, that part of a cable which is abaft the bitts,
      and so within board, when the ship rides at anchor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "bitter end":
   Thule, Ultima Thule, bottom dollar, boundary, butt, butt end,
   catastrophe, climax, completion, conclusion, consummation,
   culmination, denouement, end, end result, extreme, extremity,
   fag end, farthest bound, final result, finale, jumping-off place,
   last act, limit, nib, payoff, point, pole, stub, stump, tag,
   tag end, tail, tail end, termination, tip