鐵心 can mean “iron heart,” “steel core,” or “iron mind” in Chinese and Japanese Kanji.

This is not a common term, but I added it here since so many were looking for “iron heart.” This is almost like saying you are without emotions or feelings - a very stoic person. This is not a Buddhist trait.

鉄心石腸 is a Japanese proverb that suggests you should have the inner-strength and will as hard and steadfast as iron.

It's the Japanese way of saying, “stay strong.” This is an especially uplifting thing to say to a person in distress or recovering from a disaster. It's kind of the survivor's creed.

If you literally translate this, it means “iron will, stone guts” or “iron heart, rock-hard bowels.”

不屈 is the short form of a longer Chinese word and also a word used in Korean and Japanese to express the idea of being indomitable. It literally means “will not bend,” “will not crouch,” “will not yield,” “will not flinch,” or “will not submit.”

Note: Some will translate this as “indomitable spirit”; however, technically, there is no character to suggest the idea of “spirit” in this word.

See Also:  Tenacious / Tenacity | Perseverance / Indomitable / Invincible Fortitude | With All the Strength of Your Heart | Carry On | Undaunted

不屈不撓 means “Indomitable” or “Unyielding.”

不屈不撓 is a long word by Chinese standards. At least, it is often translated as a single word into English. It's actually a proverb in Chinese.

If you want to break it down, you can see that the first and third characters are the same. Both mean “not” (they work as a suffix to make a negative or opposite meaning to whatever character follows).

The second character means “bendable.”

The last means “scratched” or “bothered.”

So this really means “Won't be bent, can't be bothered.” I have also seen it written as “Will not crouch, will not submit.” This comes from the fact that the second character can mean “to crouch” and the last can mean “to submit” (as in “to give in” such as “submitting to the rule of someone else”). This may explain better why these four characters mean “indomitable.”

Notes:
Some will translate this as “indomitable spirit”; however, technically, there is no character to suggest the idea of “spirit” in this word.
Other translations include indefatigability, indomitableness, or unremitting tenacity.

The first two characters can be stand-alone words in Chinese.
In Japanese, this is considered two words (with very similar meanings). It's more common to see the word order flipped to 不撓不屈 in Japanese.
The same characters are used in old Korean Hanja. Just like in Japanese, the words are swapped to 不撓不屈 creating a word pronounced “불요불굴” in Korean.

See 不撓不屈

See Also:  Tenacious / Tenacity | Perseverance / Indomitable / Invincible Fortitude | With All the Strength of Your Heart | Carry On | Undaunted

鐵了心 is a Chinese title that means determined or unshakable.

This literally means “Iron Heart” or “Heart of Iron.”