'ascertainer' originates from English, specifically formed from the verb 'ascertain' + the agentive suffix '-er', where 'ascertain' ultimately comes from Old French 'acerteiner' and Latin 'certus' meaning 'certain'.

'ascertain' changed from Old French 'acerteiner' (via Middle English 'ascerten') and later the agentive '-er' was added in Modern English to form 'ascertainer'.

Initially it was related to the action 'to make certain' (from 'ascertain'); over time the derived noun came to mean 'a person who confirms or verifies' in modern usage.