A so called friend who is constantly unreliable and shady. A flake is someone who can think up an excuse for everything but their own existence. The behavior that a flake exhibits is a product of their own desire to manipulate people. A flake will find reasons to ditch a friend whenever that person wants to do something that is not on the flakes agenda. They are insidious beings they do not exhibit this behavior when you first meet them but usually much later down the line. A true flake is not the idiot that they would have you believe.
Bob secretly does not like opera, so he tells his friend to buy the tickets anyways and then does not show up. He calls the friend at the last minute and says he must study for a test. Of course it's the middle of summer vacation though, what a flake.