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Term used to describe the smell left in the bathroom after someone has taken a big dump.

Typically you will sense that the brown mist is almost tangible and it feels like it can stick to your clothing.
Guy 1: "I just went to the bathroom for a really smelly shit. Whoever goes in there next is going to be breathing in my brown mist"

Guy 2 "Really"
by Early D January 20, 2008
Term used to describe a particularly messy poo, consisting mostly of poo in its liquid form accompanied by some foul splutters of methane gas.
Brown mist is common the morning after a mess night of heavy consumption, or after eating something that upsets the stomach.
"I've got a dodgey stomach, need to do some brown mist"

guy 1: "have a nice dump?"
guy 2: "MMM! Brown Mist!"
by Stevenage steve steve August 20, 2009