'highness' originates from Old English, specifically the word 'hēahnes,' where 'hēah' meant 'high' and '-nes' was a suffix indicating a state or condition.

'hēahnes' transformed into the Middle English word 'heighnes,' and eventually became the modern English word 'highness'.

Initially, it meant 'the state of being high,' but over time it evolved into its current meaning of 'a title of honor for royalty'.