'assert' originates from Latin, specifically the word 'asserere' (also attested as 'asserere'/'assertere'), where elements such as 'ad-' meant 'to/toward' and the root related to 'join/attach' or 'bring together'.

'assert' passed into Old French (as 'asserter' or similar forms) and Middle English ('asserten'), eventually becoming the modern English verb 'assert'; the adverbial form 'assertingly' is formed by adding the present-participle/adjectival form plus '-ly'.

Initially associated with the idea of joining or bringing together and then with making a claim or laying hold of a point; over time it evolved into the current sense of confidently declaring or affirming something.