Kowabunga! That was an awesome wave!
Not beatnik, not hippy, not swahili, but early sixties, California surfer. A corrupted version of the Hawaiian "kupaianaha," which means "fantastic," and any other synonym of "wonderful" you care to choose. The word probably entered greater American consciousness through the surfer moves of the period.
For what it's worth the Teenage Mutant Ninja Turtles spoke "surfer" with a little "valley" thrown in.
“I remember back in the early 1960’s this word was used a lot, especially by beatniks and early hippies. My ninth grade english teacher told me it was Swahili for “How about that.”
Solomon: Just relax baby. This Kowabunga's gonna make you cum like a mad woman.