A dense
accumulation of feces-tainted water vapor trapped in an enclosed area.
This phenomena occurs when someone takes a long hot shower
immediately following prodigious
defecation.
it can alternatively occur if someone takes a dump while another party is showering.
"I thought she was just
showering, and I unknowingly walked into a wall of fecal fog when I opened the
bathroom door"
alternatively
"He snuck in and dropped a deuce while I was showering, it
kicked up a massive fecal fog that nearly choked us both"