"abunai(危ない)"is an adjective. It means "dangerous."
"abunaku(危なく)" is another form of "abunai," which is used in various grammatical structures, though "abunaku" itself doesn't make sense as a sentence.
For example,
"abunaku-nai" means "not dangerous."
"abunaku-naru" means "become dangerous."
"abunaku-te" means "because it's dangerous"