'nonreplaceability' is a Modern English formation composed of the prefix 'non-' (meaning 'not'), the verb 'replace' (to put back or substitute), and the suffix '-ability' (denoting capability or fitness).

'replace' entered English from Old French 'replacer' (to place again) and through Middle English became 'replace'; the prefix 'non-' has long been used in English to form negatives; the suffix '-ability' comes from Latin-derived Medieval/Old French formations (from Latin '-abilitas'). These elements combined in Modern English to form 'nonreplaceability'.

The combined formation originally and still means 'the state or quality of not being replaceable'; the sense has remained stable, used to emphasize inability to substitute or replace.