Is it ensure, insure, or assure?
mid-14c., from Old French residu (14c.), from Latin residuum "a remainder, that which is left behind," noun use of neuter of adjective residuus "remaining, left over," from residere "remain behind" (see reside).
residue res·i·due (rěz'ĭ-dōō', -dyōō')
The remainder of something after removal of parts or a part.