noun
boldness; guts; courage; gall; sangfroid
Describes a person who is unflappable, thick-skinned, or has nerves of steel, often in situations that would make others nervous. Can be used admiringly or critically.
彼は強心臓で、大勢の前でも緊張しない。
He has nerves of steel and doesn't get nervous even in front of a large audience.
あんな失礼なことを言えるなんて、強心臓だね。
You must have a lot of gall to say something so rude.
肝が据わる is a set phrase meaning 'to have guts' or 'to be unshakable', similar in meaning but more idiomatic and often used for someone who remains calm in danger.
Compound of 強 (strong) and 心臓 (heart). The figurative meaning 'boldness' comes from the idea of having a strong heart that does not waver under pressure.