'averrable' originates from Latin, specifically the word 'adverare', where 'ad-' meant 'to' (or 'toward') and 'verare' (related to 'verus') meant 'true' (with 'adverare' conveying the sense 'to make true' or 'to confirm').

'adverare' passed into Old French as 'averer' (or 'averer/aver') and then into Middle English as 'aver' meaning 'to assert or prove to be true'; the modern English adjective 'averrable' was formed by adding the suffix '-able' to 'aver'.

Initially it meant 'to make true' or 'to confirm as true'; over time the verb sense shifted to 'to assert or state as true', and the adjective 'averrable' now means 'able to be asserted or affirmed'.