'aver' originates from Old French, specifically the word 'averer', where the verb ultimately relates to Latin roots (compare Latin 'ad-' + 'verus') meaning 'to make true' or 'to prove'.

'aver' changed from Old French 'averer' (and Anglo-French forms) and entered Middle English (e.g. 'averren'), eventually becoming the modern English 'aver'.

Initially it meant 'to make true' or 'to prove to be true'; over time it shifted to the current sense 'to assert or state something to be true'.