Is it ensure, insure, or assure?
late 14c., "to overflow," from Old French redonder "overflow, abound, be in profusion" (12c.), from Latin redundare "to overflow" (see redundant). Meaning "to flow or go back" (to a place or person) is from late 14c.; hence "to rebound" (c.1500), and "to contribute to" (the credit, honor, etc.), early 15c. Related: Redounded; redounding.