2 A stomach that can hold anything (like Iron Gut, Iron belly), no matter how spicy, foul, what have you.
I think it comes from generations of people mis-hearing "Rot Gut," (which means hard liquor of such poor quality that it will rot your gut). People who knew the term "Rot Gut" would hear someone say "Rock Gut" and assume the speaker meant something like "Iron Belly," (which often makes some sense in context) and now we have both terms, though many people still say Rock gut when they mean rot gut.
His stomach can take a pounding; that's why we call him 'Old Rock gut.' Hey, Rock gut, you wanna another kettle of chili?