Oh, how I love that word.
Synonymous with "keh," the word feh can be used as a grunt, yowl, murmur, or other sound to express disdain, disrespect, disinterest, annoyance, dismissal, or any combination of the five. Often, it is a sound one "spits" out, so to speak.
"Keh," carries a similar definition; however, it usually used by one in an angier mood than "feh." It also has the potential to sound evil, whereas that would be more difficult with feh.
Compare and contrast with meh, a sound expressing complete dismissal; usually used by one ignoring another. It usually connotes bordem, or total absorption in another person or thing, as if to give no attention to another.