How Well Do You Know English Slang?
1570s, from flake + -y (2). Meaning "eccentric, crazy" first recorded 1959, said to be American English baseball slang, but probably from earlier druggie slang flake "cocaine" (1920s). Flake "eccentric person" is a 1968 back-formation from it.
The term 'flake' needs explanation. It's an insider's word, used throughout baseball, usually as an adjective; someone is considered 'flaky.' It does not mean anything so crude as 'crazy,' but it's well beyond 'screwball' and far off to the side of 'eccentric.' ["New York Times," April 26, 1964]
(Or "flakey") Subject to frequent lossage. This use is of course related to the common slang use of the word to describe a person as eccentric, crazy, or just unreliable. A system that is flaky is working, sort of - enough that you are tempted to try to use it - but fails frequently enough that the odds in favour of finishing what you start are low. Commonwealth hackish prefers dodgy.