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1.
A phrase originally coined in Lake Tahoe on the West Shore (Best Shore) by Marie Ralston. A play on words, a twist on "Charlie Sheen" meaning crazy, insane, unbelievable etc.
It's a Gnarly Sheen blizzard outside.

I took a crazy fall on the mountain today. It was Gnarly Sheen
by Abraham Bacon Von Mazeltov May 29, 2011