Is it ensure, insure, or assure?
early 14c., "a turn, a shift on duty," from Old French tour, tourn "a turn, trick, round, circuit, circumference," from torner, tourner "to turn," from Latin tornare "to polish, round off, fashion, turn on a lathe" (see turn (v.)). Sense of "a traveling around, journey" is first recorded 1640s. Tour de France is recorded from 1922. The Grand Tour, a journey through France, Germany, Switzerland, and Italy formerly was the finishing touch in the education of a gentleman.
1746, from tour (n.). Related: Toured; touring.