cast-iron stomach
The ability to eat foods that others may find unusual, unpalatable, or too spicy, without experiencing any ill effects like nausea or upset stomach. My husband has such a cast-iron stomach that he can eat a ghost pepper and feel perfectly fine afterward. You must have a cast-iron stomach if you're eating pickles with peanut butter!
cast-iron stomach
Fig. a very strong stomach that can withstand bad food or anything nauseating. If I didn't have a cast-iron stomach, I couldn't eat this stuff. Fred—known for his cast-iron stomach—ate ten whole hot peppers.