(Technically intestine, but, well.)
Literally: iron – heart – stone – gut
Alternately: Ironclad willpower; unshakeable determination; hardened resolve.
This is another invention of our friend Su Shi (蘇軾, Japanese So Shoku). It can be expressed in a bewildering variety of ways, though: the two material elements and the two bodily elements can apparently be switched around at will into almost any configuration as long as the ABAB pattern is preserved.