'asseveratory' originates from Latin, specifically the word 'asseverare', where 'ad-' (often assimilated to 'as-') meant 'to' or 'toward' and 'severare'/'severus' meant 'serious' or 'strict'.

'asseveratory' changed from the Latin verb 'asseverare' (to affirm solemnly) and the related Late Latin/Medieval Latin forms (e.g. 'asseveratus'), and through English formation by adding the adjectival suffix '-ory' it became the modern English word 'asseveratory'.

Initially it meant 'to assert or affirm solemnly' (in the verb 'asseverare'); over time the derived adjective came to mean 'serving to assert or affirm solemnly', a meaning largely consistent with the original.