You use the phrase "cast iron stomach" to describe someone who can eat all kinds of things without ever feeling sick. A person with a cast iron stomach, supposedly never experiences stomach aches.
Example in use: “It’s too spicy for me, but John will love it: He has a cast iron stomach!”
This phrase 'Cast Iron Stomach' became known around the early 1900s. Cast iron is a type of metal that is regularly used in cooking.