Is it ensure, insure, or assure?
mid-15c., "to strike against," from Middle French enterferer "to strike each other," from entre- "between" (see entre-) + ferir "to strike," from Latin ferire "to knock, strike," related to Latin forare "to bore, pierce" (see bore (v.), and cf. punch (v.), which has both the senses "to hit" and "to make a hole in"). Figurative sense of "to meddle with, oppose unrightfully" is from 1630s. Related: Interfered; interfering.