'asseveration' originates from Latin, specifically the word 'asseveratio' (from 'asseverare'), where the prefix 'ad-' (assimilated as 'as-') meant 'to/toward' and 'severus' meant 'serious' or 'stern'.

'asseveration' came into English from Latin 'asseveratio' / 'asseverare' (Medieval/Late Latin usage), with the verb form 'asseverate' and noun 'asseveration' appearing in English by the 17th century.

Initially it referred to 'the act of affirming solemnly' (in Latin/Medieval Latin); over time it has retained that sense and now means 'a solemn or emphatic declaration' in modern English.