flaky adj.

1. Made of or resembling flakes. 
2. Forming or tending to form flakes or thin, crisp fragments. 

A descriptive adjective appropriated by slang as a term of derision, "flaky" has, through excessive use, already lost its punch.  Originally "flaky" might have been used to describe an unreliable person.  Not malicious, not incompetent -- just irresponsible, in the sense of not being conscientious.

We were counting on so-and-so to follow through on a commitment but he or she, what? -- flaked off, forming thin crisp fragments?

It's not uncommon to hear an absent person called "flaky."  From the context, the listener is expected to infer that the person must have overstated his or her ability to deliver on, say, a promised investment.  In other words the flaky person did not have, could not get, or already spent the funds.  "Flaky," then, means broke -- but no more broke, of course, than the person to whom the broken promise was made.

Then, too, one might hear the advice: "The sure sign of a 'flake' is his demand for prepayment of expenses." The fraudulence implied here is probably more sinister than "flaky" deserves.

A politician, a business executive, a commentator -- anyone can be called "flaky" if his or her opinion on an issue differs from your own.

A disgruntled fan might just as likely describe a player, a coach, a sportscaster, or an umpire as flaky --  during the same game, if not the same play.

 Because of its -- well, flaky meaning, "flaky" describes nearly every person who uses the word "flaky."