Green’s Dictionary of Slang

brown stuff n.

[stuff n. (5); the colour]

1. as the colour of drink or drugs.

(a) [1900s] (Aus./US) whisky.

(b) [1930s–40s] (US drugs) opium.

(c) [1970s] (US drugs) Mexican heroin.

2. [1950s+] (also brown baby) excrement, also used fig., i.e. ‘the shit’.