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.
condition produced by aerial oxidation of unsaturated fat present in foods and other products, marked by unpleasant odour or flavour. When a fatty substance is exposed to air, its unsaturated components are converted into hydroperoxides, which break down into volatile aldehydes, esters, alcohols, ketones, and hydrocarbons, some of which have disagreeable odours. Butter becomes rancid by the foregoing process and by hydrolysis, which liberates volatile and malodorous acids, particularly butyric acid. Saturated fats such as beef tallow are resistant to oxidation and seldom become rancid at ordinary temperatures.