'assertive' originates from Latin, specifically the word 'asserere' (past participle 'assertus'), where the prefix 'ad-' (often assimilated to 'as-') meant 'to' and the root 'serere' meant 'to join or bind'.

'assertive' changed from Latin 'assert-' (from 'asserere'/'assertus') into Late Latin and through Old French/Medieval Latin influences; the modern English adjective was formed by adding the adjectival suffix '-ive' to the verb stem 'assert-'.

Initially it was connected to the action of 'claiming' or 'affirming' (to assert); over time it developed into the adjectival sense 'tending to assert' or 'expressing assertion' and the broader sense of 'confidently stating'.