"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.