Is it ensure, insure, or assure?
mid-14c., from Old French mote "mound, hillock, embankment; castle built on a hill" (12c.; Modern French motte), from Medieval Latin mota "mound, fortified height," of unknown origin, perhaps from Gaulish mutt, mutta. Sense shifted in Norman French from the castle mound to the ditch dug around it. As a verb, "to surround with a moat," early 15c.