“Kusala comes from “ku” +” sala,” where “ku” refers to “kunu” or kilesa or evils, and “sala” means “to get rid of.””
I first saw this interpretation of the etymology of kusala in the way in Pure Dhamma. I’m not saying it is wrong. I have never learned the Pali language professionally. But I read a book that explains the etymology of the word kusala before.
The book explained: kusala comes from “kusa” + “la”. “Kusa” originated from a plant that grows in India. “La” means to cut. Kusa is used in Indian rituals. It is also a tough and sharp plant. So when someone tries to cut the kusa, he should be careful and concentrate. Just like one who tries to fulfill goodness and wholesome should be skillful. So, kusala means skillfulness, goodness, and wholesomeness. The basis for kusa+la interpretation is Atthasalini. Atthasalini is the commentary of Dhammasangani. It was written by Buddhaghosa.
I think the interpretation of Pure Dhamma is valid in that it better reveals the overall context of Dhamma: The way to decontaminate(Majjhimā Paṭipadā). I think it also supports the fact that he wasn’t Ariya.