Etymology of the exclamation 「よし」

Question

What is the etymology of the exclamation 「よし！」?

I mean the one that's made before people decide to take some kind of action (such as standing up, or starting to do the dishes).

Answer 1

It's from 良し, which is the dictionary form of 良い ("good") in classical Japanese. It's just like saying "Good" or "Okay" before making a decision in English. よし is now considered a lexicalized interjection, but 良し is still used in modern Japanese in the sense of "excellent".