When a hot chick has become such an ungrateful bitch, that she doesn't appear to be hot anymore. Hence the "fog"...
"Rachel has been such a lying, whorish bitch lately, the bitch fog is starting to set in and I can barely see when I'm around her!"
The stanky smell that emits from an unclean female's crotch. Similar to nutsack musk on a male - both can be smelled from a distance on a hot summer day, or while either is doing the walk of shame the next morning.
Friend 1: What IS that rank stench!?
Friend 2: (points at a hooker) Eww, it's gotta be her bitch fog!