A person whose social and psychological outer layer slips away when under pressure. They lack integrity, resilience and robustness. For their protection they depend upon unrealistic and imaginative constructs.
(I was called out on my use of the word "flake" recently and was challenged to define it. I came up pretty much with the above definition and then went through all the other definitions that people have supplied to the UD for it. I was struck by the consistency of them - especially the self-interest and unreliability. We know who these people are ...)