Source : Webster's Revised Unabridged Dictionary (1913)

Mightiness \Might"i*ness\, n.
   1. The quality of being mighty; possession of might; power;
      greatness; high dignity.

            How soon this mightiness meets misery. --Shak.

   2. Highness; excellency; -- with a possessive pronoun, a
      title of dignity; as, their high mightinesses.

Source : WordNet®

mightiness
     n : physical strength [syn: {might}, {power}]