Fug is slang for a thick, smelly atmosphere such as that found in a poorly ventilated room. It can also mean a hot, close or smoky state of space, as in a crowded bar. The word is often used in conjuction with discussion of air pollution (fug being associated with the haze caused by chemicals in the atmosphere).
Example:
The room was filled with a fug of smoke that one could hardly see through.
Some such uses of fug:
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http://www.peugeotclub.org/peugeot/archive/99_8/0542.html
http://www.wired.com/wired/archive/9.02/baylis_pr.html