Sisu is a Finnish concept and cultural construct that is described through a combination of various English terms including stoic determination, grit, bravery, resilience, and hardiness and is held by Finns themselves to express their national character. It is generally considered to not have a literal equivalent in English.
Sisu is a grim, gritty, white-knuckle form of courage that is typically presented in situations where success is against the odds. It expresses itself in taking action against the odds and displaying courage and resoluteness in the face of adversity, in other words, deciding on a course of action and then sticking to that decision even despite repeated failures. It is in some ways similar to equanimity, with the addition of a grim kind of stress management.