An acronym standing for "Feet Ass & Nuts".
The term is used to describe the combined reek of foot odor, flatulence and scrotum sweat. The term is believed to have origins in the US Navy, where close quarters necessitated by life aboard a ship and rationing of fresh water led to a breakdown of bodily hygiene, leading to the terrible collective odor known as F.A.N.. The acronym has since found use throughout the US military and wherever stinkyness of men living in close quarters is to be found.
"Holy crap! It smells like F.A.N. in this barracks!"