'assertedly' originates from Latin, specifically the word 'assertare' (a frequentative formation related to 'asserere'), where 'ad-' meant 'to/toward' and 'serere' meant 'to join/put together'.

'assertedly' changed from Middle English forms derived from Old French 'asserter' (and Latin 'asserere'/'assertare') and eventually became the modern English verb 'assert' and the adverbial form 'assertedly' through the addition of the past-participial suffix '-ed' and the adverbial suffix '-ly'.

Initially related to the Latin sense of 'affirming' or 'maintaining', it evolved into the modern adverbial meaning 'in the manner of being asserted; as asserted'.