the.com/highness

means A formal title of respect used for members of a royal family (as in 'Your Highness' or 'His Royal Highness'), standing in for their actual name.

from Built from 'high' plus the suffix '-ness,' which turns an adjective into a noun of state or quality — so 'highness' literally means the condition of being high, i.e. elevated in rank. English borrowed the whole pattern of using abstract-quality words as royal titles (compare 'majesty,' 'grace,' 'excellency') from medieval European court custom, where addressing a ruler by an exalted abstraction, rather than by name, was itself a mark of deference. The title crept in gradually during the medieval and early modern periods as courts formalized layers of address for different ranks of royalty.