flake - ... 2: [informal] a crazy or eccentric person.
flaky - ... 2: [informal] crazy or eccentric: flaky ideas about taxes.
[informal] (of a device or software) prone to break down; unreliable
Urban Dictionary (link in #2):
flake (238 thumbs up, 16 thumbs down) -
n. An unreliable person; someone who agrees to do something, but never follows through.
1. John called in sick to work again today. He's such a flake.
2. Mary said she would do the research for our project, but it's been a week and she hasn't done a thing. She's such a flake.
I don't think it has anything to do with being stingy or selfish.
To me, flaky just means nutty, ditzy, not having one's head tightly screwed on, having a screw loose. A flake is a (mildly) crazy, irrational, or eccentric person, a nutcase, someone you can't rely on to act logically or predictably.
So my impression is that the definition as not reliable or dependable, if it can be said to exist, is just a subset of that broader sense. In fact, it looks as if the young people at Urban Dictionary (many of whom often don't seem to read much or know much English apart from slang) may just not have known the real meaning, so they jumped to the conclusion that flaky inherently means unreliable, because nutty people are usually also unreliable. However, their usage may eventually win out.
@laalaa: Thanks, that's helpful. Next time, though, could you please not just copy a link with no comment; take the 5 seconds to copy the text, so we don't all have to duplicate your effort just to find out if it's even relevant, and so that everyone can see it. Remember, some people have dial-up modems or restricted internet access from work.