A flake is a weak, indecisive, uninformed, unmotivated kind of person who might start something but doesn't have what it takes to finish it. To flake out means to quit in the middle, to give up, to run away from a responsibility, to hide from danger. A more formal word would be "dilettante" or "frivolous". To flake out means the action of shirking responsibility or quitting in the middle or abandoning a position you should be holding.