Cast Iron Stomach noun phrase
If someone has a cast iron stomach, they have a strong stomach that can digest unusual food and even bad food without being ill.
Jimmy must have a cast-iron stomach, when he drank milk with lime juice and felt perfectly fine afterward.
You must have a cast-iron stomach if you eat this stuff.
To have a strong stomach against bad or unusual food
This idiom became widespread around the early 1900s since cast-iron is a type of metal that commonly used in creating cooking utensils. Pans that are made with cast-irons are tough and resilient hence, making the meaning of the idiom.
To keep optimistic or have hope that something positive will occur and simultaneously prepare for a negative outcome
All I can do is hoping for the best and preparing for the worst.