Highness

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Old English hēanes (see high, -ness).

wiktionary

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From Middle English hyghnesse; equivalent to high +‎ -ness.

etymonline

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highness (n.)

Old English heanes; see high (adj.) + -ness. Meaning "royalty, excellence, nobility" is early 13c.; Your Highness as a form of address to English royalty is attested from c. 1400.