Is it ensure, insure, or assure?
c.1300, "a step backward;" late 14c., "act of retiring or withdrawing; military signal for retiring from action or exercise," from Old French retret, noun use of past participle of retrere "draw back," from Latin retrahere "draw back, withdraw, call back," from re- "back" (see re-) + trahere "to draw" (see tract (n.1)). Meaning "place of seclusion" is from early 15c.; sense of "establishment for mentally ill persons" is from 1797. Meaning "period of retirement for religious self-examination" is from 1756.
early 15c., "to draw in, draw back, leave the extremities," from retreat (n.) and in part from Old French retret, past participle of retrere. Meaning "to fall back from battle" is mid-15c. Related: Retreated; retreating.