The American Heritage® Dictionary of the English Language, 4th Edition
n. A heavy, stale atmosphere, especially the musty air of an overcrowded or poorly ventilated room: "In spite of the open windows the stench had become a reeking fug” ( Colleen McCullough).
n. A heavy, musty, and unpleasant atmosphere, usually in a poorly-ventilated area.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. (British informal) an airless smoky smelly atmosphere
Word Usage
"I think this book should have been called An Abundance of Fugs, mainly because the word fug (or some variation thereof) is used about 7500 times (not that I counted)."