It’s the epitome of surfer-dude slang: totally gnarly dude!
Thinking about the word today though, it seemed a strange fit for surfing slang. Surely, I thought, it’s related to gnarled, as in knobbly, rough, or twisted. Not how you’d describe a wave really, is it?
No, but the word leant itself well to drifting towards meaning difficult or dangerous, and a gnarly wave is therefore a difficult one, that provides an appropriate challenge for a surfer.
Tubular, another stereotypical piece of surfer slang, is a little more straightforward, and is one I’m proud to admit I figured out myself in a moment of random thought. A really difficult, big wave with a crest curving forward so much that it almost touches the surface of the water would be quite literally tubular in its resemblance to a tube!