Green’s Dictionary of Slang

real thing, the n.

also the real stuff, the real article

1. [19C+] of people, circumstances or objects, the genuine article.

2. [mid-19C+] (Irish) the finest whisky, often with a suggestion that it has been obtained illicitly; fine brandy.

3. [1940s–50s] (US drugs) heroin.

4. [1970s] marijuana.