About the Origin of the Kamehameha
The Kamehamaha comes from the name of a Hawaiian king named King Kame Hame Ha. Theoretically, Akira Toriyama and his wife were on vacation in Hawaii and Akira Toriyama's wife thought of this there. This may not be true, its just a theory.
About Akira Toriyama
Akira Toriyama is the creator of Dragonball/Z/GT and is Japanese. He also created Dr. Slump which was the attempt of Dragonball, but Dragonball was the revised form with different characters, but same theme.