'backness' originates from Old English, specifically the word 'bæc' meaning 'back' combined with the noun-forming suffix '-ness' (from Old English '-nes(s)e' meaning 'state or quality').

'back' changed from Old English 'bæc' and, with the Middle English use of the suffix '-ness', produced the modern English noun 'backness'.

Initially, it meant 'the state or quality of being at the back or rear'; over time it also acquired a specialized technical sense in phonetics referring to 'the degree to which a vowel is articulated with the tongue toward the back of the mouth'.