Green’s Dictionary of Slang

(US) of an individual, second-rate.

2005		Urban Dict. 2 May 🌐 Plug Adjective. A word used to describe someone who is completly and entirly [sic] useless.
2016		Jeezy ‘Let Em Know’ 🎵 I don't fuck around with cap niggas / See the problem is there way too many plug niggas.