One theory of origin

“According to the Honolulu Star-Bulletin,[2] prevailing local lore credits the gesture to Hamana Kalili of Laie,[3] who lost the three middle fingers of his right hand while working at the Kahuku Sugar Mill. Kalili was then shifted to guarding the sugar train, and his all-clear wave of thumb and pinkie is said to have evolved into the shaka as children imitated the gesture”