rancid

Rancid means smelling of fat that has gone off. Rancid butter, gone rancid, a rancid smell.

It applies specifically to fats and oils, where oxygen and light break them down and produce the stale sharp smell — which is why oils are sold in dark bottles and kept cool. Figuratively it describes something morally sour. Note that rancid food is unpleasant rather than usually dangerous, unlike food carrying bacteria.