To be very calm and steady under pressure; to have strong nerves or guts; to remain composed and unflappable in stressful situations.
彼女は事故現場でも肝が据わっていて、周りの人を落ち着かせた。
She stayed calm at the accident scene and helped steady the people around her.
大勢の観客の前でスピーチしても、彼は肝が据わっているように見えた。
Even speaking in front of a large audience, he seemed to have nerves of steel.
子どもが泣き出したが、看護師は肝が据わっていてすぐに処置を始めた。
The child started crying, but the nurse remained composed and began treatment right away.