I am not Finnish, but having spent most of my adult life in one of the counties bordering to Finland, the word sisu has not escaped me, though a proper understanding of what it means requires a cultural insight which I probably do not possess. Store norske leksikon explains sisu as such:
Sisu er finsk og betyr utholdenhet, seig kraft, kamplyst, både legemlig og sjelelig.
ETYMOLOGI fra finsk sisu, egentlig ‘det indre’
Sisu is Finnish and means perseverance; raw, prolonged power; will to fight, both corporeal and spiritual.
Etymology: from Finnish sisu, original meaning ‘the inner’
What are good ways to translate sisu to Latin? I am especially looking for attested, conceptualised forms from any period. Finnish Latinists are especially encouraged to answer, as they presumably possess the cultural background to fully conceptualise what is meant by the term.