'asseverate' originates from Latin, specifically the word 'asseverare', where 'ad-' (a variant assimilated to 'as-') meant 'to' or 'toward' and 'severare' (from 'severus') meant 'serious' or 'grave'.

'asseverare' in Latin passed into Medieval/Neo-Latin as 'asseverare'/'asseveratus' and then into English as 'asseverate' via scholarly and legal/clerical Latin usage, becoming the modern English verb 'asseverate'.

Initially, it meant 'to make a serious or solemn declaration' and over time it has retained that sense, now used as 'to affirm solemnly or earnestly'.