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1.
a uniquely Finnish quality;
the word used to typify the Finnish spirit;
a concept that is at the heart of how all Finns view themselves;
a certain feature or value considered by Finns to be typically Finnish

- endurance, resilience, tenacity, determination, perseverance

- an inner reserve of diligence, capacity, the ability to face head-on and always overcome

- craziness: the recklessness that inspires a person to take on something in the face of incredible odds

- bravery, empowerment, inner strength
Gritting your teeth, continuing to fight against an overwhelming enemy, clearing a forest with your bare hands, continuing on to win a race even after falling.

Sisu means that you finish what you start, you don’t quit in the middle of a job, and you don’t whine.

"It doesn’t take sisu to go to the North Pole; it takes sisu to stand at the door when the bear is on the other side."
by Martha's_Cellmate November 11, 2004
 
2.
Finnish principle of high power of will which should make you to achieve anything. Characteristics: Determination, courage, persistence (etc.)
In Finland we have three things called sisu, sauna and Sibelius and that's what we are.
by Esko Pasko May 16, 2004
 
3.
A typically Finnish term, meaning strengh, perseverance and resilience. Could be taken as similar in meaning to the English saying, 'Just do it.'
Sisu is not often used in other countries, but is very important to the Finns, as they feel that it is part of what makes them, them.
Niilo: 'I'm so tired, I just can't finish this marathon!'
Heikki: 'Come on bro! You're a Fin! You've got Sisu, right?'
Niilo: 'I guess so! Finnish pride FTW!'
by EmilyMoony October 08, 2012