The word evolved from “kawabonga,” a word used by the character Chief Thunderthud on the United States children’s television program Howdy Doody in the 1950s. “Kawagoopa” was an all-purpose exclamation of the Tinka Tonka Indians, the fictitious tribe of Princess Summerfall Winterspring and Chief Featherman, and “kawabonga” was the equivalent exclamation of Chief Thunderthud, who was of the Ooragnak Tribe. By the 1960s, surfers who had grown up watching Howdy Doody adopted it for surfing use, turning it to “cowabunga.”

[ Cowabunga - Wikipedia, the free encyclopedia ]

Why, you may ask, did I look this one up? Oddly the reason is also on the Wikipedia page:

Cookie Monster of Sesame Street, began to exclaim “Kowabunga!” in the 1970s. (Note the “K” spelling. He used it during a Letter of the Day segment as an example of a “K” word. When challenged by Prairie Dawn that it was not a word, he replied: “Kowabunga too a word. It esoteric, but it poetic in its own right.”)

So Cookie Monster is a surfer? Or just loved the Howdy Doody show?