Ever had a cast iron Dutch oven, Skillet, or griddle acquire a smell that may not be so favorable? If so, it's probably gone rancid. It can happen from time to time due to how people tend to clean their cast iron, or perhaps more common, how long it's been since they last cooked in it.
After cooking a tasty meal in your cast iron and properly cleaning it, it's a good idea to rub a small amount of cooking oil, crisco, or cast iron conditioner into the piece and briefly heat it back up. This will help protect your patina and protect your cast iron. However, cooking oil and crisco do have a shorter shelf life than conditioner but all can go rancid at some point if your cast sits too long.
Here's a quick minute-and-a-half video from Gary House of how to simply clean rancid cast iron.