The file
A nonsense word from a '50s kids' show that surfers adopted as a war cry, that the Turtles took national, that Bart Simpson said on a T-shirt. Three generations of kids, one word, nobody sure what it means.
Michelangelo said it every episode. That's the whole reason anyone born after 1980 knows it.
Sources: Wiktionary — Cowabunga · Wikipedia — Cowabunga