'assertorially' originates from English, formed from the adjective 'assertorial' plus the adverbial suffix '-ly'. 'Assertorial' itself is built from the verb 'assert' and the adjectival suffix '-orial'.

'assert' comes from Latin (via Late Latin/Old French) 'asserere'/'assertere' (to declare, affirm); this passed into Middle English as 'asserten' and produced the adjective 'assertorial' (relating to asserting), which later took the adverbial suffix to become 'assertorially'.

Initially the root meant 'to affirm or declare'; over time it yielded adjectival and adverbial forms used to describe the manner of making such affirmations—today 'assertorially' means 'in an asserting/definitive manner'.