Is it ensure, insure, or assure?
early 15c., from Middle French interjection (Old French interjeccion, 13c.), from Latin interiectionem (nominative interiectio) "a throwing or placing between," noun of action from past participle stem of intericere, from inter- "between" (see inter-) + -icere, comb. form of iacere "to throw" (see jet (v.)).
A brief exclamation, often containing only one word: “Oh!” “Gee!” “Good grief!” “Ouch!”