'asserter' originates from Latin, specifically the word 'assertor', where the prefix 'ad-' (in the historical form) meant 'to/toward' and the root 'serere' (related form) meant 'to join/put together' (used in the sense of affirming or declaring).

'asserter' changed from Medieval Latin 'assertor' (meaning 'one who asserts') and passed into Middle English via Old French/Medieval Latin influences, eventually becoming the modern English noun 'asserter'.

Initially, it meant 'one who affirms or defends a claim', and over time it has retained that basic sense of 'a person who asserts' in modern English.