Green’s Dictionary of Slang

1. (US drugs) phencyclidine; thus wack back n., a user of phencyclidine who becomes violent.

1985		T.R. Houser Central Sl.
1996		Bynum & Thompson Juvenile Delinquency 290: Wack, liquid or juice. P.C.P.
2003		Microgram Bulletin XXXVI:8 182: On the street, PCP is commonly referred to as Angel Dust, Hog, Ozone, Rocket Fuel, Shermans, Wack, Crystal, and Embalming Fluid.

2. see whacko n.