'unseparated' originates from the Old English prefix 'un-' meaning 'not' attached to 'separated'. 'Separated' ultimately comes from Latin 'separatus', the past participle of 'separare', where 'separare' meant 'to sever, to make apart.'

'unseparated' developed by adding the negative prefix 'un-' to the past-participle/adjective form 'separated' (Middle English and earlier forms such as 'unseperated' appear in historical texts). 'Separated' itself came into English via Old French from Latin 'separare'/'separatus'.

Initially it simply meant 'not separated' and that core meaning has remained stable; usage has extended into technical contexts (e.g., chemistry, biology) to describe components that have not been divided or isolated.