'assessably' originates from Latin, specifically the Late Latin word 'assessare' (frequentative of 'assidēre' / 'assidēre'), where the prefix 'ad-' (in 'assidēre') meant 'to, toward' and 'sedēre' meant 'to sit.'

'assessably' changed from Latin into Old French and Middle English forms: Late Latin 'assessare' → Old French/Middle English verbs (e.g. 'assesser' / 'assessen') → English verb 'assess', then with the adjective-forming suffix '-able' to 'assessable' and adverbial '-ly' to 'assessably'.

Initially, related Latin forms meant 'to sit beside (as a judge) / to evaluate'; over time the sense shifted to 'determine value or amount', and modern 'assessably' means 'in a way that can be assessed.'