1. （脂肪食物）变坏的，发臭的
If butter, bacon, or other oily foods are rancid, they have gone bad and taste old and unpleasant.

1. smelling of fermentation or staleness

Synonym: sour

2. (used of decomposing oils or fats) having a rank smell or taste usually due to a chemical change or decomposition

e.g. rancid butter
rancid bacon