Is it ensure, insure, or assure?
1640s, from Latin rancidus "rank, stinking, offensive" (also source of Italian rancido, Spanish rancio), from rancere "be spoiled or rotten," of unknown origin. German ranzig is from French rancide. Related: Rancidness.
rancid ran·cid (rān'sĭd)
Having the disagreeable odor or taste of decomposing oils or fats.