Green’s Dictionary of Slang

hard stuff n.

[hard adj. (1) + stuff n.]

1. [late 18C–1940s] money in the form of coins, as opposed to notes.

2. [mid-19C+] spirits, as opposed to beer.

3. [1950s+] (drugs, also harder stuff, hard shit) drugs like narcotics, rather than tranquillizers, cannabis etc.

In phrases

hit the hard stuff (v.)

[20C+] to drink (spirits rather than beer or wine).