attest (verb) – to bear witness to or certify; to affirm to be correct, true, or genuine
BREAKDOWN: AT- (to) + TEST- (witness)
— an attestant is one who bears witness, certifies, or confirms
This word is built on the root TEST-, which means witness.
Common TEST- words include contest, detest, and protest.
More esoteric TEST- words include intestacy and obtest.