IPA: /əˈtɛstər/
KK: /əˈtɛstər/
A person who confirms or verifies something, often by signing a document to show that it is true or valid.
The attestor signed the contract to confirm that both parties agreed to the terms.
Attestor is formed from "ad-" (meaning to) and "testari" (meaning to witness or testify). The word refers to someone who witnesses or testifies to the truth of something.
Think of someone who is 'to witness' ('ad-' means to, and 'testari' means to witness) — that's why an attestor is a person who testifies.