interjection: An expression of surprise, joy, or enthusiasm.
The word was the cry of Chief Thunderthud, a character in the children’s television program Howdy Doody. The word was later adopted by surfers. It was popularized by its use on the animated show Teenage Mutant Ninja Turtles. Earliest documented use: 1954.
“Malaysia reported its first case of a cow giving birth to triplets last week.”
Alina Simon; Cowabunga! It’s Triplets; New Straits Times (Kuala Lumpur, Malaysia); Oct 24, 2010.