60s SoCal surfer slang popularized by artist Matt Goening's animated skateboarding cartoon character Bart Simpson, son of Homer "Doh!" Simpson, on the still running ― 20+ years ― "The Simpsons" TV show.
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.