Is it ensure, insure, or assure?
c.1200, "continue, persevere," from Anglo-French suer "follow after, continue," from Old French sivre, later suivre "pursue, follow after," from Vulgar Latin *sequere "follow," from Latin sequi "follow" (see sequel). Sense of "start a lawsuit against" first recorded c.1300, on notion of "following up" a matter in court. Sometimes short for ensue or pursue. Related: Sued; suing.