Is it ensure, insure, or assure?
1867, coined to serve as opposite of synonym, from Greek anti- "equal to, instead of, opposite" (see anti-) + -onym "name" (see name (n.)). Perhaps introduced to English in the book "Synonyms and Antonyms" (1867) by the Ven. C.J. Smith, M.A.
UNDER the head of Synonyms and Antonyms, Archdeacon Smith arranges words which form an antithesis to one another. The word "antonym" is, we believe, a new formation but useful. ["Journal of Sacred Literature," July 1867]French antonyme (1842), German antonym (by 1859) are older. The un-Greek alternative counterterm has been left to fade.