Is it ensure, insure, or assure?
late 14c., "remove" something, "quarantine, isolate" (someone); "excommunicate;" also intransitive, "separate oneself from," from Old French sequestrer (14c.), from Late Latin sequestrare "to place in safekeeping," from Latin sequester "trustee, mediator," noun use of an adjective meaning "intermediate," which probably is related to sequi "to follow" (see sequel). Meaning "seize by authority, confiscate" is first attested 1510s. Alternative sequestrate (v.) is early 15c., from Latin sequestratus. Related: Sequestered; sequestering.