na-adjective, noun
invulnerability; immortality; insensitivity to pain
Describes a person or being that cannot be killed, wounded, or hurt. Often used figuratively for someone who seems unbreakable or never gets sick. The 'insensibility to pain' nuance is less common and usually appears in contexts of extreme resilience.
彼は不死身の男と呼ばれている。
He is called the invulnerable man.
あの選手は怪我をしてもすぐ復帰する。まるで不死身
That player comes back right away even after an injury. It's as if he's invincible.
無敵 means 'invincible' or 'unrivaled' in a competitive sense, whereas 不死身 focuses on the inability to be killed or physically overcome.
Compound of 不 (not), 死 (death), and 身 (body). The literal meaning is 'undying body.' The word has been used in Japanese for centuries to describe mythical or legendary figures who cannot be killed.