to find out definitely; learn with certainty or assurance; determine: to ascertain the facts.
2.
Archaic. to make certain, clear, or definitely known.
Origin: 1400–50; late Middle English, variant of assertain, acertain < Middle French acertain- (tonic stem of acertener to make certain), equivalent to a-a-5 + certaincertain
a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison.
the offspring of a zebra and a donkey.
a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question.
early 15c., "to inform, to give assurance," from O.Fr. acertener "to assure" (13c.), from a "to" + certain "certain" (see certain). Modern meaning of "to find out for sure by experiment or investigation" is first attested 1794.