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

'assever' changed from Medieval Latin 'asseverare' (or 'adseverare') into English via later Latin/vernacular usage and appeared in Early Modern English as a learned verb; it eventually settled as the modern English 'assever'.

Initially it meant 'to make serious or to confirm solemnly'; over time it came to mean more specifically 'to assert or affirm earnestly', a usage preserved in modern English.