Knotted and twisted in appearance, or slang for something extreme, challenging, or impressive.
From 'gnarl' (a knot in wood), which comes from Old English Germanic roots; popularized as slang by surfers in the 1970s-80s.
The same word describes both the twisted wood of old trees and awesome surfing waves because surfers used 'gnarly' to mean anything wild and hard to handle—just like knotted wood is difficult to work with.
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