A plug is a person who sells weed or other drugs. To run off on the plug means to run away from a drug dealer, with the product, without paying them the agreed upon amount. This is currently being done as a "fun prank" to video tape and post online, often backfiring and ending in disaster.
Made famous by the rapper Plies song "Ritz Carlton". Now it is a dance often performed by everyone even Nfl players after scoring. A very disrespectful way to treat a drug dealer. Basically I'll take yo drugs not give you no damn money, and if ya ass stupid enough to sell to me again I'll do it again
Beware, running off on the wrong plug will likely get you shot.