what is " cast iron stomach ' ?
"I don't know how you can eat that spicy food. You must have a
cast iron stomach!"

Question

what is " cast iron stomach ' ?
 "I don't know how you can eat that spicy food.  You must have a 
  cast iron stomach!"

Answer

cast-iron stomach is a very strong stomach that can withstand bad food or anything nauseating

Answer

A person with a cast iron stomach can eat or drink anything without any ill effects
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.

Answer

It means that this person can eat any type of food without being sick.