The Collaborative International Dictionary of English v.0.48:

Bitterly \Bit"ter*ly\, adv.
   In a bitter manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "bitterly":
   abominably, acerbically, acerbly, acidly, acridly, acrimoniously,
   agonizingly, awfully, baldly, balefully, bitingly, blatantly,
   brashly, caustically, confoundedly, corrodingly, corrosively,
   cruelly, cuttingly, damnably, deadly, deathly, deucedly,
   distressingly, dolorously, dreadfully, egregiously, excessively,
   excruciatingly, exorbitantly, extravagantly, flagrantly,
   frightfully, grievously, hardly, hellishly, horribly, improperly,
   incisively, inexcusably, infernally, inordinately, intolerably,
   keenly, lamentably, miserably, mordaciously, mordantly, nakedly,
   openly, painfully, penetratingly, piercingly, piteously,
   rancorously, sadly, scathingly, scorchingly, sharply, shatteringly,
   shockingly, something awful, something fierce, sorely, stabbingly,
   staggeringly, tartly, terribly, torturously, trenchantly,
   unashamedly, unbearably, unconscionably, unduly, unpardonably,
   witheringly, woefully