Chiefly in French or continental European contexts: a person of high rank, a nobleman or noblewoman. Frequently as a title, usually with possessive and capital initial: ‘Highness’. Sometimes ironic or humorous. Compare altezza .
Mid 16th century. From Middle French altesse, title or form of address for a high-ranking person, probably from Italian altezza or perhaps from Spanish alteza, with remodelling of the suffix after French -esse.