Is it ensure, insure, or assure?
1650s, "to select (someone) for a group or club by a vote of members," from Latin cooptare "to elect, to choose as a colleague or member of one's tribe," from com- "together" (see com-) + optare "choose" (see option (n.)). For some reason this defied the usual pattern of Latin-to-English adaptation, which should have yielded *cooptate. Sense of "take over" is first recorded c.1953. Related: Co-opted; co-opting.