'irreplaceableness' originates from English, formed by adding the noun-forming suffix '-ness' to 'irreplaceable' (itself composed of the negative prefix 'ir-' + 'replaceable'). 'ir-' derives from Latin 'in-' (negative), and 'replace' comes via Old French 'replacer' (to put back) from Latin elements meaning 'again' (re-) and 'place/put'.

'replace' changed from Old French 'replacer' into Middle English forms such as 'replacen' and eventually the modern English 'replace'. The negative prefix 'in-' became 'ir-' before 'r' (as in 'irreplaceable'), and later the suffix '-ness' was added to form the abstract noun 'irreplaceableness'.

Initially the roots conveyed the concrete action 'to put back' (replace) and simple negation 'not' (ir-); over time these combined to describe the abstract property 'not able to be replaced', which is the current meaning of 'irreplaceableness'.