Green’s Dictionary of Slang

(orig. surfing) an excl. of pleasure, victory (over the waves) etc; latterly an excl. of surprise.

1954		Mad mag. Dec. 13: Kowabunga, Buffalo Bill ... me lookin’ for Clarabella.
1966		Scientific American Dec. 148: Cartoon columns from California papers make argot such as ho-dad and cowabunga clear to the uninitiated.
1968		‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 10: cowbonga – Surfer’s yell of victory over a hard to ride wave.
1972		Dahlskog Dict. Contemp. and Colloq. Usage.
1986		Detroit Free Press (MI) 24 Sept. 23/2: Cowabunga! [...] a new syndicate show called ‘Couch Potato’.
1997		G.V. Higgins Change of Gravity [ebook] ’[O]ver a hundred and twenty thousand dollars [...] That’s fairly serious money.’ ‘Cowabunga,’ she said. ‘On the salary a clerk makes?’.
1999		Christian Science Monitor 30 Dec. n.p.: The exclamation ‘Cowabunga!’ appears to have originated with Chief Thunderthud on ‘The Howdy Doody Show’ in 1954. It passed into surfing slang as an exclamation of triumph in the 1960s, then became a rallying cry for the Teenage Mutant Ninja Turtles in the 1980s and more recently one of Bart’s catchwords on ‘The Simpsons’.
2001		OnLine Dict. of Playground Sl. 🌐 Cowabunga n. (1) It’s like... ‘Oh, my God!’ ‘Wow!’ ‘I’m impressed!’, While used in the ‘Teenage Mutant Ninja Turtles’ books/movies/etc., the expression goes back at least to the late 1940s with the ‘Howdy Doody’ television program, where it was used quite often as an expression of surprise. (2) common use is as a ‘shout’ made just before a charge... similar to Geronimo!