Derived form the sport of cow throwing. The participant in the event.
Bung as in to throw, makes the entrant a 'Bunga'.
The Peanut Gallery: Cowabunga!
Surfer: Cowabunga, dude!
Of course, surfers picked word up ― as Vermont Ferret wrote ― from Chief Thinderthud's greeting and his peanut gallery's response on the old time (1950s) children's TV show "Howdy Dowdy," an eponym for its cowboy manikin. Back then, on old corny Westerns, an Amerindian's greeting was stereotypically, "How!" to be rejoined with a "How!" So How morphed into Cow and an "abunga" was added for silly kid ― or those with Peter Pan syndrome ― fun.