Green’s Dictionary of Slang

1. (drugs) especially strong marijuana; marijuana mixed with phencyclidine.

1969		R.R. Lingeman Drugs from A to Z (1970) 237: superpot marijuana which has been soaked in alcohol, dried, then smoked. Another method is to inject the alcohol into a marijuana cigarette with a hypodermic needle. The resulting cigarette is said to be more potent in its hallucinogenic effects.
1977		S.N. Pradhan Drug Abuse.
1981		D.E. Miller Bk of Jargon 339: supergrass, superweed: Marijuana cut with PCP, formaldehyde, or other strong sense distorters; also, any extremely high-potency marijuana.
2001		ONDCP Street Terms 20: Super grass — [...] marijuana with PCP; marijuana [...] Super pot — Marijuana.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 181/2: supergrass n. 2 very potent marijuana, marijuana of very high quality.
2003		Microgram Bulletin XXXVI:8 182: PCP combined with marijuana is referred to as Killer Joints, Super Grass, Fry, Lovelies, Wets, and Waters. Today, PCP joints are often referred to as ‘dippers’ because users dip the joints into a PCP-laced liquid referred to as ‘water.’.

2. phencyclidine.

1979		H. Feldman et al. Angel Dust 124: The large number of street names it has been accorded over the years: [...] super weed.
2001		ONDCP Street Terms 20: Super grass — PCP [...] Super weed — PCP.