'attestable' comes from the verb 'attest' plus the adjectival suffix '-able'. 'Attest' ultimately originates from Latin, specifically the word 'attestari', where 'ad-' meant 'to/toward' and 'testari' meant 'to call as witness' (from 'testis', 'witness').

'Attest' passed into English via Old French (attester) and Middle English (attesten); the modern adjective 'attestable' developed by adding the suffix '-able' (from Latin/French) to the verb 'attest'.

Initially related to 'bearing witness' or 'giving testimony'; over time it broadened to mean 'capable of being proved or certified', which is the current sense behind 'attestable'.