鉄てっ石せき心しん

て

っ

せ

き

し

ん

Meanings

1

noun

Literary or formal term for an extremely firm, unshakable will, like iron or stone.

iron

石stone

心heart, mind, spirit

不ふ屈くつ
不屈 means 'indomitable' or 'unyielding' and is more common in modern Japanese, while 鉄石心 is a more literary, metaphorical expression.
鉄てつの意い志し
鉄の意志 is a more direct and common phrase for 'iron will', whereas 鉄石心 is a compact, classical compound.

Compound of 鉄 (iron), 石 (stone), and 心 (heart/mind), literally 'iron-stone heart', expressing an unbreakable will.