Is it ensure, insure, or assure?
late 13c., from Anglo-French anuier, Old French enoiier, anuier "to weary, vex, anger; be troublesome or irksome to," from Late Latin inodiare "make loathsome," from Latin (esse) in odio "(it is to me) hateful," ablative of odium "hatred" (see odium). Earliest form of the word in English was as a noun, c.1200, "feeling of irritation, displeasure, distaste." Related: Annoyed; annoying; annoyingly. Middle English also had annoyful and annoyous (both late 14c.).