'assertor' originates from Latin, specifically the Late Latin word 'assertor', where the root is related to the verb 'asserere' meaning 'to declare or affirm'.

'assertor' passed into Middle English (often via Old French/Middle French forms such as 'assertour') and eventually became the modern English noun 'assertor'.

Initially, it meant 'one who affirms or declares', and over time it has retained that basic meaning as 'a person who asserts'.