1. unreliable, liable to act in an unconventional way
a flaky person
Something that is described as flaky breaks off easily into small layered pieces. So you could have flaky skin after getting sunburnt for example, or eat a flaky pastry. A person is someone who is considered unreliable and eccentric. Someone who promises to show up somewhere and then never arrives, or cancels at the last minute could be called flaky ie they break off plans easily. This is an adjective used in informal contexts.