The Brief: A plug or "the plug" is slang for someone who knows where to get drugs. "To plug" as a verb means to advertise something.
A plug can be a drug dealer or someone who knows a drug dealer. The term is often used in reference to marijuana but can include all types of drugs.
The term is commonly used in rap and R&B songs.
Plug, plug, plug, I’m the plug for her –Chance the Rapper, “I’m the One”
There is a song called “I’m The Plug” by Drake and Future, which is about being a plug/drug dealer.