Is it ensure, insure, or assure?
early 14c., "constructor of military engines," from Old French engigneor, from Late Latin ingeniare (see engine); general sense of "inventor, designer" is recorded from early 15c.; civil sense, in reference to public works, is recorded from c.1600. Meaning "locomotive driver" is first attested 1832, American English. A "maker of engines" in ancient Greece was a mekhanopoios.