Green’s Dictionary of Slang

(drugs) phencyclidine.

1979		H. Feldman et al. Angel Dust 124: The large number of street names it has been accorded over the years: [...] KJ.
1981		D.E. Miller Bk of Jargon 338: KJ: PCP.
2001		ONDCP Street Terms 13: KJ — PCP.