it is a suffix, which shows in this case the turning of a noun into an adjective.
as the example in wikipedia is shown: kukka (“flower”) → kukikas (“flower-patterned”) or ääni (“noise”) → äänekäs (“noisy”) or voima (“power”) → voimakas (“powerful”). For further information see https://en.wiktionary.org/wiki/-kas#Finnish (Am I allowed to do that...?)
‘Sisu’ in Finnish means strength, perseverance in a task that for some may seem crazy to undertake, almost hopeless.
Apparently it's a word that's used in English
Borrowed from Finnish sisu, documented in English since at least 1940.'
1940 January 8, “Northern Theatre:Sisu”, in Time Magazine:
Last week the Finns gave the world a good example of sisu by carrying the war into Russian territory on one front while on another they withstood merciless attacks by a reinforced Russian Army.
1999, Norma J. Livo & George O. Livo, The Enchanted Wood and Other Tales from Finland, →ISBN, page 5:
People living in northern climates need plenty of sisu.
2016, Angela Duckworth, Grit: The Power of Passion and Perseverance, →ISBN:
We agreed there was a pressing need for a systematic investigation of sisu, how Finns think about it, how it is propagated.
2016, Katie Falkowski, GRIND: Greatness Rises in Each New Day: Get up, Show up, and Grind, →ISBN:
Life requires all of us to have a certain level of sisu when things get tough, which they inevitably will.
Definitely grit. I'm a Finn, and in many use cases grit alone is a good translation. Personally I think that the words grit, guts and fortitude together cover the complete meaning of sisu, but all three are rarely needed together to convey the meaning in specific contexts.
I also think that fortitude is an underused English word while describing sisu; I'd guess that that's because it's not in the English vocabulary of second language speaking Finns.
Brave is not the same as "with sisu". Please read the tips section for this lesson: https://www.duolingo.com/skill/fi/basics_2/tips-and-notes