Rancid describes the disgusting smell and taste of fats or oils that have gone bad. When butter sits out too long or cooking oil gets old, it turns rancid: it develops a sharp, sour, unpleasant odor that makes you wrinkle your nose and want to throw it away immediately.

The word comes from how fats break down chemically when exposed to air over time. Fresh nuts taste pleasant and slightly sweet, but rancid nuts taste bitter and smell off. Old potato chips that have been open for months might taste rancid. Milk can go beyond just sour: it smells truly foul.

You'll sometimes hear people describe other bad smells as rancid, even when they're not actually about spoiled fats. Someone might describe the smell of a locker room or garbage can as rancid when they mean it's particularly disgusting. The word carries that sense of something spoiled and revolting in a way that makes your stomach turn.