This is the traditional “Hang Loose” sign you flash on appropriate occasions; generally with a flip of the wrist or shaking back & forth motion. I can mean “right on brah!”, “no worries”, “that’s cool”, “be cool”, “relax, brah” and any other number of things depending on the situation. Like if someone stops to allow you to enter the highway from a driveway you might flash them the shaka and a way of saying “thank you”. BTW, “brah” is a more modern contraction of the Hawaiian “Bruddah” which is pidgin for “brother”. What’s “pidgin”? https://www.youtube.com/watch?v=O7X9AAeDCr4
Back in the old sugar mill days a worker at the Kahuku mill named Hamana Kalili got the three middle fingers of his right hand cut off in a mill accident. No longer able to work his job, the boss at the mill gave him the job of guarding the mill and facilities including the sugar mill train. The mill and train cars were a popular place for kids to play, and the mill owners feared someone would get hurt; so there was an effort to keep the kids out. Before there was a guard, the kids would sneak in through the cane fields and play around the mill half the night. After Mr. Kalili was put on as guard, it became a game for the kids to antagonize him and not get caught. So they wouldn’t have to yell a warning to each other (and reveal their location) when Mr. Kalili was approaching, they would mimic what his hand looked like, shaking their fist in the air with their thumb and pinky finger extended for the other kids to see. The shaking motion is what gave the name “shaka” to the gesture. True story.