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noun; A term for a small primitive structure found all over ski hills, in which people go to smoke marijuana. Derived from the word "hillbilly" used to describe the stoned Montanan mountain folk that originally built these ancient saunas billions of years ago to survive the extinction of the dinosaurs. Usually built from from logs and tarp, these structures protect users from wind in order to provide an optimum lighter operating environment.
Person 1: "Alright, I'm beat. Lets hit the sauna."
Person 2: "Hot sauna or Billy Sauna? Either way I'm gonna pass out."
by SlateDamage April 27, 2011
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