D defs.my
Entry 2 senses Webster, 1913

Mightiness

· Might·i·ness · IPA
01 n. The quality of being mighty; possession of might; power; greatness; high dignity.
  1. 1.
    The quality of being mighty; possession of might; power; greatness; high dignity.
    “How soon this mightiness meets misery.” Shak.
  2. 2.
    Highness; excellency; -- with a possessive pronoun, a title of dignity; as, their high mightinesses.