One way to hold this word: a knot in old wood. Where the grain of a tree twists back on itself, it leaves a hard lump — a gnarl. Gnarly is what a thing looks like when it is full of them: bent out of line, rough to the hand, and impossible to cut straight through. Every other use of the word is that lump moved somewhere else. Onto a pair of old working hands. Onto a problem you cannot start at one end of. Onto a wave that could knock you over. And, because a thing like that is and thrilling in the same moment, onto anything you want to call extreme — terrible or magnificent.
From the word’s history
From gnarl, the knot in a piece of wood, plus -y: literally 'full of knots'. The slang uses were popularised by US surf culture in the 1970s, which is how a word about wood came to be shouted at waves.
由 gnarl（木頭上的節瘤）加上 -y 構成，字面就是「滿是節瘤的」。來源指出那些俚語用法是 1970 年代美國衝浪文化帶紅的，這也解釋了一個原本講木頭的字，後來怎麼會被拿去對著大浪喊。
Wiktionary, "gnarly"
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