'asseverative' originates from Latin, specifically the word 'asseverare', where the prefix 'ad-' (in assimilated form 'as-') meant 'to' and 'severare' (related to 'severus') meant 'serious' or 'stern'.

'asseverative' changed from Medieval Latin 'asseverativus' and passed into English via the verb form 'asseverate' (Middle English), eventually becoming the modern adjective 'asseverative'.

Initially it referred to the act of solemnly affirming or asserting; over time it has come to describe something that serves to assert or that expresses strong affirmation (i.e., 'serving to affirm strongly').