Source : Webster's Revised Unabridged Dictionary (1913)

Rueful \Rue"ful\, a.
   1. Causing one to rue or lament; woeful; mournful; sorrowful.

   2. Expressing sorrow. ``Rueful faces.'' --Dryden.

            Two rueful figures, with long black cloaks. --Sir W.
                                                  Scott.
      -- {Rue"ful*ly}, adv. -- {Rue"ful*ness}, n.

Source : WordNet®

ruefulness
     n : sadness associated with some wrong done or some
         disappointment; "he drank to drown his sorrows"; "he
         wrote a note expressing his regret"; "to his rue, the
         error cost him the game" [syn: {sorrow}, {regret}, {rue}]