Is it ensure, insure, or assure?
mid-14c., from Old French perpetuel "without end" (12c.) and directly from Latin perpetualis "universal," in Medieval Latin "permanent," from perpetuus "continuous, universal," from perpetis, genitive of Old Latin perpes "lasting," probably from per- "through" + root of petere "to seek, go to, aim at" (see petition (n.)). Related: Perpetually. Perpetual motion is attested from 1590s.