Green’s Dictionary of Slang

(drugs) mint or poss. parsley leaves impregnated with phencyclidine (PCP) for smoking.

1979		H. Feldman et al. Angel Dust 124: The large number of street names it has been accorded over the years: [...] mint leaf.
2001		ONDCP Street Terms 15: Mint leaf — PCP. [...] Mint weed — PCP.