'unjoined' is formed from the negative prefix 'un-' (Old English 'un-' meaning 'not') + 'joined', the past participle of 'join'. 'Join' originates from Old French 'joindre' from Latin 'iungere' (variant 'iug-') where the root meant 'to join' or 'yoke'.

'join' passed from Latin 'iungere' into Old French as 'joindre', then into Middle English as 'join'. The negative prefix 'un-' was attached in Middle English to form 'unjoin', with the past/past-participle form becoming 'unjoined' in Modern English.

Initially the elements simply conveyed 'not' + 'joined' (i.e., 'not joined'); this basic meaning has remained stable, giving the modern sense 'not connected' or the past action 'separated'.