Green’s Dictionary of Slang

1. (drugs) to take dexedrine.

1967		T. Southern ‘Blood of a Wig’ in Red Dirt Marijuana (1973) 221: I finished about ten, dexed, and made it to the office.
1980		The Ruts ‘Staring at the Rude Boys’ 🎵 The rude boys are dexing to some heavy, heavy ska.

2. to take dextromethorphan (DXM).

1996		alt.drugs.psychedelics (Usenet newsgroup) 20 Feb. 🌐 Also, if you want to have a BLAST when you’re dexing, try some nitrous during your peak … it’s far out. Happy dexing.
2003		USA Today 29 Dec. (Factiva) A1: Kids don’t have to drink entire bottles of goopy cough syrup to go ‘Robotripping’ or ‘Dexing.’ Pills such as Coricidin HBP Cough & Cold tablets – known as ‘Triple C’s’ – offer far more potent doses of DXM with less hassle.
2019		www.verywellfamily.com/a-teen-slang-dictionary 10 Mar. 🌐 Dexing - Abusing cough syrup.