"Great!"
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.