Green’s Dictionary of Slang

flake n.1

[SE flake, a thin broad piece peeled or split off from the surface of something]

1. [1910s+] (drugs) cocaine, spec. pieces that are smaller than average.

2. [1950s] (drugs) heroin.

3. [1970s+] (drugs) in pl., phencyclidine.

4. [1980s] (Aus.) shark meat, esp. as sold in fish ’n’ chip shops.