adverb (r)

• powerfully or vigorously(adv.all)

  Example:
  He strove mightily to achieve a better position in life.
  

  source: wordnet30
• (Southern regional intensive) very; to a great degree(adv.all)

  Example:
  They rejoiced mightily.
  

  Synonym:
  mighty, powerful, right
  

  source: wordnet30
• In a mighty manner; with might; with great earnestness; vigorously; powerfully.(adverb)

  source: webster1913