Green’s Dictionary of Slang

1. a form of hallucinogen [abbr. of either serenity, tranquillity and peace, or play on scientifically treated petroleum, a gasoline additive].

1968		L. Wolf Voices from the Love Generation 74: I’m going to take STP. I’m kind of afraid of it but I’m going to take it.
1971		Harper’s Mag. Sept. 63: [He] is off everything at the moment—pot, hash, speed, acid, STP, mesc, psyl, coke, scag—because ‘It’s all a bummer, man; I got my head together’.
1999		Guardian Guide 14–20 Aug. 59: His LSD-crazy pals decide to help the poor girl by giving her [...] a dose of STP.

2. phencyclidine [a mistaken use of sense 1].

2001

ONDCP Street Terms 20: STP — PCP.