SISU is an untranslatable Finnish term that blends RESILIENCE, tenacity, persistence, determination, perseverance and sustained, rather than momentary, courage: the psychological strength to ensure that regardless of the cost or the consequences, what has to be done will be done.

It originates from the word sisus, meaning “intestines” or “guts”; Daniel Juslenius, author of the first Finnish-language dictionary in 1745, defined sisucunda as the place in the body where strong emotions live. In a harsh environment and with powerful neighbours, it was what a young nation needed.
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More prosaically, it has helped Finns get through a lot of long, lonely, dark and freezing winters, building in the process one of the wealthiest, safest, most stable and best-governed countries in the world. It is not all good, of course. Sisu can lead to stubbornness, a refusal to take advice, an inability to admit weakness, a lack of compassion.
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Taken from the article in The Guardian