'unauthorisedly' originates from the negative prefix 'un-' + 'authorised' (the adjectival form of 'authorise') + the adverbial suffix '-ly'. 'Authorise' comes via Old French 'autoriser' from Late Latin 'auctorizare', where 'auctor' meant 'originator, author'.

'unauthorisedly' developed by adding the prefix 'un-' (meaning 'not') to the adjective 'authorised' and then adding the Old English/Old Germanic-derived adverbial suffix '-ly', producing the modern adverb 'unauthorisedly'.

Initially the components meant 'not' + 'given authority' and this has remained consistent; over time the formed adverb has come to mean simply 'in a manner lacking official permission' in modern usage.