noun
A literary four-character compound meaning an extremely firm, unshakable will, like iron and stone. Often used in formal or written contexts.
彼は鉄石心腸の持ち主で、どんな困難にも屈しない。
He has an iron will and never gives in to any hardship.
More common and neutral expression for 'iron will'. 鉄石心腸 is more literary and emphatic.
A yojijukugo (four-character idiom) composed of 鉄 (iron), 石 (stone), 心 (heart/mind), and 腸 (guts/spirit), literally meaning 'iron-stone heart-guts', conveying an unyielding spirit.