In a great degree; very much. A sense scarcely to be admitted but in low language.--Todd's Johnson.
An ass and an ape conferring grievances; the ass complained mightily for want of horns, and the ape for want of a tail.--L'Estrange.
·adv To a great degree; very much. II. Mightily ·adv In a mighty manner; with might; with great ear...
Webster's Dictionary of the English Language