It is a rather versatile expression, as it can be used in amazement, excitement, full of surprise, anger, disapproval, etc.
Right now, I would say it for lack of anything better to say. I feel apathetic, and rather tired, so BAH!
User whscubana on Urban Dictionary described it well: "a word used to express feelings of tiredness, boredom, or frustration."
I would have to disagree with that on a few terms though; I wouldn't consider it a word, as it doesn't really have a meaning. It is an expression. Also, that description just covers one small portion of that expressions extent.
However, his example of "bah" used in a sentance is quite amuzing: "Bah. I have a boring life AND I can't open a can with a conventional can opener."
This definition, courtesy of al describes how I feel now: "An expression of apathy stemming from frustration."
I know I was frustrated recently, and not for really any certain reason, but I am now just feeling apathetic.
But yea, I think thats about as much thought as I can give to my new expression of choice. Until I rant again, bah I tell you! BAH!!!!!!!1