Green’s Dictionary of Slang

1. (US prison) to get psychologically prepared to start a (gang) fight [? SE flex].

1992		Bentley & Corbett Prison Sl. 90: Plex [...] To become bristled and ready to fight.
2003		C-Note ‘Hold It Down Pt. 2’ 🎵 If them boys wanna plex, put a strap to they dome.
2010		T.I. ‘Everything On Me’ 🎵 Now with all due respect, I don’t front, I don’t plex.

2. (US black) to show off.

1987		LL Cool J ‘Do Wop’ 🎵 The girlies want sex, the fellas try to plex.
1992		Ultramagnetic MCs ‘Bust The Facts’ 🎵 Because it’s time to plex more and rhyme fantastic.

3. (US) to show disrespect, to slander [ety. unknown].

2001

(con. 1975–6) E. Little Steel Toes 19: Don’t plex on me.

4. (US prison) to be anxious or nervous [? SE perplex].

1997		E. Little Another Day in Paradise 97: Why’s a white boy plexin’ over being called “boy” – it ain’t no thing.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Plexin: To be anxious or nervous. (NY).