[1980s+ Teenagers; said to have originated among 1970s surfers, describing a dangerous wave; this sense persisted among skateboarders until at least 1988]
jargon
/nar'lee/ Both obscure and hairy. "Yow! - the tuned assembler implementation of BitBlt is really gnarly!" From a similar but less specific usage in surfer slang.
[Jargon File]
(1996-09-17)