First time I heard about. I like that they have a word for it and its fairly short, I might just start using it myself. I speak almost 4 languages and always fascinated by concepts that can't be directly translated.
Also like that there is a scale applied to it and it can be a negative thing:
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[...] there can be too much sisu, and according to the survey answers this leads to bull-headedness, foolhardiness, self-centeredness and inflexible thinking.
[1] https://en.wikipedia.org/wiki/Sisu