'inseparableness' originates from Latin, specifically the word 'inseparabilis', where 'in-' meant 'not' and 'separare' meant 'to separate'.

'inseparabilis' changed into Medieval Latin/Old French 'inseparable', which entered Middle English as 'inseparable'; English later formed the noun by adding the suffix '-ness' to create 'inseparableness'.

Initially it meant 'not able to be separated', and over time it has retained this core meaning while also being used figuratively for close relationships or conceptual unity.