'assertress' originates from English, formed from the verb 'assert' plus the feminine agentive suffix '-ress' (from French suffix '-esse'), where 'assert' itself ultimately comes from Latin 'asserere'.

'assert' passed into Middle English from Old French/Latin (compare Latin 'asserere'), and the feminine suffix '-ress' (modeled on agent-nouns like 'actor'/'actress') was added in Modern English to coin 'assertress' as a female agent noun.

Initially it referred simply to a female person who asserts or declares; over time it remained rare and is now largely archaic or nonstandard, with 'assertor' or neutral terms preferred.