'attestor' originates from Latin via Old French/Anglo-French forms of the verb 'attest', specifically from Latin 'attestari', where 'ad-' meant 'to' and 'testari' meant 'to give witness, testify'.

'attestor' changed from Middle English/Anglo-Norman forms such as 'attestour' (from Old French 'attester') and was formed in English by adding the agentive suffix '-or' to the verb 'attest', eventually becoming the modern English noun 'attestor'.

Initially, the root meant 'to bear witness' or 'to testify', and over time the noun came to mean 'one who bears witness or formally certifies'—a meaning that has largely been retained in modern usage.