Teenage Mutant Ninja Turtles : how did cowabunga start? where did it start? what culture?

Unknown, but was popularized (as Kowa-Bunga) by Chief Thunderthud on the American children's show Howdy Doody in the late 1940s or early 1950s, and became associated with American surfing culture. By the early 1980s, it was used as the catch-phrase of Cookie Monster on the American television show Sesame Street, and gained popularity with a broader audience after its use in the American cartoon Teenage Mutant Ninja Turtles in the late 1980s and early 1990s. (For further treatment, see here.)