In the general population, kook means weird, off-beat type person.
In the surf population its meant a shit talker who doesn't back his claims up on the water for a while, and then came to mean generally lame person. Then it spread into skate language.
In the end, words are collections of sounds created by people that are coded with a meaning. Are any words really insults? Should we be offended by any individual word out of context? Probably not, but there are many examples of times we are.