My father attended a british public school in the 1940's where "bluff" was slang for fart. I mention this only because alot of the boys slang dated back centuries. i.e., they called the electric lights dips, as the gas lights had been called before, as candles had been called since the time of the founder.
Shouldn't definition 2 specify a false expression of self-confidence? In poker, if I have a good hand, I may very well want to make a genuine show of confidence in order to reduce the number of players in the hand and thereby increase the chances that my good hand will "hold up." That fits the second definition, but it's not a bluff. It's only a bluff if I don't really expect my hand to win, and I pretend to be confident anyway. I'd also add a related definition of bluff as any empty threat. Example: "Told my girl we'd have to break up / Thought that she would call my bluff" (from the song "Big Girls Don't Cry")