Sisu

Sisu is a Finnish term loosely translated into English as strength of will, determination, perseverance, and acting rationally in the face of adversity. However, the word is widely considered to lack a proper translation into any other language. Sisu has been described as being integral to understanding Finnish culture. The literal meaning is equivalent in English to "having guts", and the word derives from sisus, which means something inner or interior. However sisu is defined by a long-term element in it; it is not momentary courage, but the ability to sustain an action against the odds. Deciding on a course of action and then sticking to that decision against repeated failures is sisu. It is similar to equanimity, except the forbearance of sisu has a grimmer quality of stress management than the latter. The noun sisu is related to the adjective sisukas, one having the quality of sisu.

However, while "sisu" is problably too deeply rooted in the Finnish culture to have an absolute translation into any other language, there are bound to be related concepts in other cultures and the corresponding term could be considered as a translation. In English the nearest equivalent is "bottle", or in former years "bottom" (in Cockney rhyming slang *bottle" means the same thing, but you have to be English to understand that idea.) "Bottle" is similarly believed to be untranslatable - the nearest is "sisu".