adverb (r)

• with bitterness, in a resentful manner(adv.all)

  Example:
  She complained bitterly.
  

  source: wordnet30
• indicating something hard to accept(adv.all)

  Example:
  He was bitterly disappointed.
  

  source: wordnet30
• extremely and sharply(adv.all)

  Example:
  It was bitterly cold.
  

  Synonym:
  bitingly, bitter, piercingly
  

  source: wordnet30
• In a bitter manner.(adverb)

  source: webster1913