Is it ensure, insure, or assure?
early 13c. (transitive) "force back, drive back," from Old French reculer "to go back, give way, recede, retreat" (12c.), from Vulgar Latin *reculare, from Latin re- "back" (see re-) + culus "backside, bottom, fundament." Meaning "shrink back, retreat" is first recorded c.1300; and that of "spring back" (as a gun) in 1520s. Related: Recoiled; recoiling.
c.1300, "retreat," from Old French recul "recoil, backward movement, retreat," from reculer (see recoil (v.)). Meaning "back-kick of a firearm" is from 1570s.