early 15c., "to inform, to give assurance" (a sense now obsolete), from Anglo-French acerteiner, Old French acertener "to assure, certify" (13c.), from a- "to" (see ad-) + certain "sure, assured" (see certain). Meaning "find out for sure by experiment or investigation" is first attested 1794. Related: Ascertained; ascertaining.
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