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).
The room was filled with a fug of smoke that one could hardly see through.
Some such uses of fug: