Source : Webster's Revised Unabridged Dictionary (1913)

Bitterness \Bit"ter*ness\, n. [AS. biternys; biter better + -nys
   = -ness.]
   1. The quality or state of being bitter, sharp, or acrid, in
      either a literal or figurative sense; implacableness;
      resentfulness; severity; keenness of reproach or sarcasm;
      deep distress, grief, or vexation of mind.

            The lip that curls with bitterness.   --Percival.

            I will complain in the bitterness of my soul. --Job
                                                  vii. 11.

   2. A state of extreme impiety or enmity to God.

            Thou art in the gall of bitterness, and in the bond
            of iniquity.                          --Acts viii.
                                                  23.

   3. Dangerous error, or schism, tending to draw persons to
      apostasy.

            Looking diligently, . . . lest any root of
            bitterness springing up trouble you.  --Heb. xii.
                                                  15.

Source : WordNet®

bitterness
     n 1: a feeling of deep and bitter anger and ill-will [syn: {resentment},
           {gall}, {rancor}, {rancour}]
     2: a sharp and bitter manner [syn: {acrimony}, {acerbity}, {jaundice}]
     3: the taste experience when quinine or coffee is taken into
        the mouth [syn: {bitter}]
     4: the property of having a harsh unpleasant taste [syn: {bitter}]