'apartness' is formed in English by adding the suffix '-ness' to 'apart'. 'apart' originates from Middle English 'a part(e)', ultimately from Old French 'a part', and from Latin elements 'ad' (meaning 'to, toward') + 'partem/pars' (meaning 'part').

'apart' came from Middle English 'a part(e)' (literally 'at part'), itself from Old French 'a part', which traces back to Latin elements such as 'ad' + 'partem'; the modern noun 'apartness' developed later by attaching the descriptive noun-forming suffix '-ness' to 'apart'.

Originally related to the idea of being 'at a part' or 'to a part' (physical separation), it evolved to express the general condition of separateness or emotional/social distance as in the modern 'apartness'.