'assumer' originates from Latin, specifically the verb 'assumere,' where the prefix 'ad-' (often assimilated to 'as-') meant 'to/toward' and 'sumere' meant 'to take.'

'assumere' passed into Old French as 'assumer' and into Middle English as 'assumen'/'assumen(e)'; the modern English verb 'assume' derives from these forms, and the agent noun 'assumer' was later formed in English by adding the suffix '-er' to the verb.

Initially it meant 'to take up or take to oneself'; over time it broadened to include 'to take on (a role)' and 'to suppose,' and the noun 'assumer' has come to mean 'one who takes on' or, less commonly, 'one who presumes.'