Green’s Dictionary of Slang

(US drugs) a pure marijuana cigarette, as opposed to one that has been laced with cocaine or another drug.

1995		L. Stavsky et al. A2Z.
1997		G. Sikes 8 Ball Chicks (1998) 54: He lit a joint, sticking it under my nose to make sure I knew it was weed — ‘herbal,’ as he called it, not a ‘primo’ laced with cocaine.