When it’s kept aggressively cold indoors, by use of air conditioning, to overcompensate for the outside heat. The opposite of a regular sauna, where it is too hot inside but feels unbearably cold once you exit.

If it is 100 degrees and you are wondering whether you should have brought a light jacket to your work, school, etc., you have likely stumbled into a devil’s sauna.
My parents get upset if I touch the thermostat; they like to keep a devil’s sauna.
by linguistical June 1, 2021
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When it's too hot to cuddle but you do it anyway
My Gf took one for the team it was a helluva sauna snuggle
by małpa July 15, 2021
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