Is it ensure, insure, or assure?
mid-15c., Scottish, "postpone, defer," from Middle French superceder "desist, delay, defer," from Latin supersedere "sit on top of, stay clear of, abstain from, forbear, refrain from," from super "above" (see super-) + sedere "to sit" (see sedentary). In Scottish law, a judicial order protecting a debtor. Meaning "displace, replace" first recorded 1640s. Related: Superseded; superseding.