"It is not good."
Maybe not so much "one word" but I think if you call it a "set" you're on the right track. よく (yoku) in this sentence comes from the adjective よい(yoi) or いい (ii), meaning good. ない (nai) is word to negate something. よくない as a set means "not good". Unlike English adjectives, Japanese -i adjectives change forms to convey negation or the past tense. -i adjectives drop the final "i" and add "ku" + "nai" to form the negative version of the adjective.