'nonseparable' originates from Modern English, specifically formed from the negative prefix 'non-' (from Latin 'non') and the adjective 'separable' (from Latin 'separabilis' / 'separare'), where 'non-' meant 'not' and 'separare' meant 'to separate'.

'separable' came into English via Old French 'separable' from Latin 'separabilis' (from 'separare'). The compound form 'non-separable' was created in Modern English by adding the prefix 'non-' to 'separable' and later became written as the single word 'nonseparable'.

Initially, it meant 'not able to be separated', and this basic meaning has remained largely unchanged into modern usage.