Classical Japanese ク and シク adjectives – 酸し base form?

I've been reading on the history of sushi and learned that the origin of the word is a terminal conjugation of the classical adjective meaning "sour" (酸し). So I started reading about adjectives in classical Japanese and I understand that instead of い and な adjectives, they had ク and シク adjectives (which later turned into い and しい adjectives), but I'm a bit confused with how they conjugate. In this case, would the unconjugated form of 酸し be 酸く? Or is it conjugated from the nominal form 酸 (a sour taste)?