Sisu

Saw this great explanation, part of a bit longer article, from Finlandia University

To the Finnish people, sisu has a mystical, almost magical meaning. Sisu (pronounced - see'-soo) is a unique Finnish concept. It is a Finnish term that can be roughly translated into English as strength of will, determination, perseverance, and acting rationally in the face of adversity.

Sisu is not momentary courage, but the ability to sustain that courage. It is a word that cannot be fully translated. It defines the Finnish people and their character. It stands for the philosophy that what must be done will be done, regardless of cost.

Sisu is an inherent characteristic of the Finnish people. You might call it backbone, spunk, stamina, guts, or drive and perseverance. It is a measure of integrity that surpasses the hardship and sees through to the end.