Etymology
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gnarly (adj.)

"knotted and rugged," c. 1600, from gnarl (see gnarled) + -y (2). Picked up 1970s as surfer slang to describe a dangerous wave; it had spread to teen slang by 1982, where it meant both "excellent" and "disgusting."

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Definitions of gnarly

gnarly (adj.)
used of old persons or old trees; covered with knobs or knots;
Synonyms: gnarled / knotted / knotty / knobbed
From wordnet.princeton.edu