There is a lot of debate on the origins of the shaka, but I've heard one consistent story that revolves around someone named Hamana Kalili. Basically, he lost his middle 3 fingers working in a sugar mill, and then he worked as a guard on the sugar train. As time passed as he worked there "his all clear" wave evolved into what we know as the shaka. Nowadays, it has quite the range of meanings and you can see it used all over the world.
 
High quality print of an original drawing! 11x14" keeps it true to the original sketch!