Learn how to say Highnesses correctly in English with this tutorial pronunciation video.
Oxford dictionary definition of the word highness:
1 (His/Your etc. Highness) a title given to a person of royal rank, or used in addressing them:
I am most grateful, Your Highness
2 [mass noun] the quality of being high:
the highness of her cheekbones
Old English hēanes (see high, -ness)