Sisu
Etymology
From Finnish, derived from sisus, meaning "interior" or "guts" — literally the insides of a person or thing. The word thus grounds courage in the body’s core, much as English speaks of "guts" or "having it in you." Finns often treat sisu as a defining national trait, and while the concept is genuine and old, that framing of it as uniquely Finnish is partly a matter of cultural pride.
noun · /SEE-soo/
Stoic determination and grit — the resolve to keep going through hardship long after ordinary persistence would give out.
What it names
The reserve of stubborn inner strength you draw on precisely when a situation looks hopeless and quitting would be reasonable.
Example
Down two sets and cramping, she found the sisu to drag the match into a fifth.
Related idea
Resilience is often the capacity to recover and bounce back after a blow; sisu is the harder-edged will to push forward straight through the blow while it is still landing.