Is it ensure, insure, or assure?
Old English styrne "severe, strict," from Proto-Germanic *sternijaz (cf. Middle High German sterre, German starr "stiff," störrig "obstinate;" Gothic andstaurran "to be stiff;" Old Norse stara; Old English starian "to look or gaze upon"), from PIE root *ster-, *star- "be rigid" (see sterile).
c.1300, "hind part of a ship, steering gear of a ship," probably from Old Norse stjorn "a steering," related to styra "to guide" (see steer (v.)). Or the word may come from Old Frisian stiarne "rudder," which is also related to steer (v.).