With a slightly different spelling - "cowabunga" it was used as an exclamatory in "Teenage mutant ninja/hero turtles" in the late 80s early 90s. My son, who was small at the time, used the word repeatedly.
It was also used in the West Coast surfing community around the same time. I've heard it.
A quick check in Google suggests that it was originally heard in a 1950's American sitcom "Howdy Doody".
I've no idea if it extends back beyond that (and I'd never heard of "Howdy Doody, which seems to be rather a blessing).