Cowabunga it is

An expression used to accept a challenge, wage war or defend one's honor by unspecified but extreme/chaotic measures.
In all instances it is a simple way of indicating that come what may, your decision is set in stone.

An expression used to indicate anger, suprise, or frustration, and very often associated with a macro of Michaelangelo of the Teenage Mutant Ninja Turtles with a wide mouthed stare on his face.

(exclamation): Originating as a greeting by Chief Thunderthud (the only Indian ever to have a moustache) to the Peanut Gallery on the 50's television program "The Howdy Doody Show", the term was later adopted by surfers in the 60's.

When you pull a girls bloody tampon out and swing it like a nunchuk and hit her in the face with it.

An exclamation of rejoice from the early 1990s. Usually uttered by teenagers wearing neon colors. Brought to popularity by the ever-so-famous Teenage Mutant Ninja Turtles.

Basically the male version of the shocker. When a female forms a cowabunga sign with her hand and simultaneously jerks him off while sticking a thumb in and out of his butt using the SAME hand.

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.