Green’s Dictionary of Slang

flake n.2

[flaky adj.]

1. [1950s+] (orig. US, also flakeout, flako) a boring, unappealing, incompetent, undesirable person.

2. [1950s+] (US) an eccentric, crazy person.

3. [1960s+] (US) a disappointment or failure.

4. [1990s+] a worthless or second-rate type.