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

'asseveration' changed from Medieval/Latin 'asseveratio' and entered Middle English as forms like 'asseveracion', eventually becoming the modern English word 'asseveration'.

Initially, it meant 'to affirm or state seriously/solemnly', and over time it has kept this core sense, now used chiefly for a solemn or emphatic declaration.