Basically, if something like “woah that wave was gnarly dude” comes to mind, you have the right idea (or at least the popularized one). But its meaning changed again over the years.

Harrison Novak, surfer dude and star quarterback, has Agoura feeling gnarly Harrison Novak of Agoura is a 5-foot-7 quarterback who passed for 394 yards and six touchdowns against Calabasas.