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1829, "knotted and rugged," from gnarl (see gnarled) + -y (2). Picked up 1970s as surfer slang to describe a dangerous wave; it had spread in teen slang by 1982, where it meant both "excellent" and "disgusting."
[1980s+ Teenagers; said to have originated among 1970s surfers, describing a dangerous wave; this sense persisted among skateboarders until at least 1988]