indomitable

peräänantamaton, lannistumaton Incapable of being subdued, overcome, or vanquished.
1902, w:A. E. W. Mason|A. E. W. Mason, The Four Feathers, ch. 1:

Personal courage and an indomitable self-confidence were the chief, indeed the only, qualities which sprang to light in General Feversham.

But he was a youth of indomitable spirit, strong and agile as a wild cat.

Nobody came on to the movie camera—wrapped it in a bear hug and wrestled it to submission—like Betty Hutton. They called this 40s singer-actress "the Blitzkrieg blond" . . . . She was indomitable, unstoppable.