The file
A word that meant bad and good at the same time, which is how you knew it was surf slang. A gnarly wipeout was awful and a gnarly ride was the best day of your life.
Sean Penn's Spicoli said it in Fast Times and the rest of the country said it for a decade. Say it now and someone will ask if you're doing a bit.
Sources: Wiktionary — Gnarly · Wikipedia — Fast Times at Ridgemont High