'assertor' originates from Latin, specifically the Late Latin word 'assertor', where the verb 'asserĕre' (or related Latin stems) meant 'to affirm' or 'to proclaim'.

'assertor' changed from Late Latin 'assertor' into usage in medieval Latin/Anglo-Norman contexts and eventually entered modern English as 'assertor' with little change in form.

Initially, it meant 'one who declares or affirms', and over time it has largely retained that meaning, though its use has become somewhat specialized (formal or legal contexts).