Source : Webster's Revised Unabridged Dictionary (1913)

Bitterly \Bit"ter*ly\, adv.
   In a bitter manner.

Source : WordNet®

bitterly
     adv 1: in a resentful manner; "she complained bitterly" [syn: {with
            bitterness}]
     2: indicating something hard to accept; "he was bitterly
        disappointed"
     3: extremely and sharply; "it was bitterly cold"; "bitter cold"
        [syn: {piercingly}, {bitingly}, {bitter}]