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.
To not only skip out on your partner in crime so dat he gets da blame for your "combined" transgressions, but you also abscond with whateverill-gotten booty dat said hapless cohort helped you to "acquire"!
In da "Adventures Of Huckleberry Finn" tale, da King and da Duke had both secretly planned to bilk da other out of da sack of gold --- i.e., each one of said selfish 'n' shameless shysters possessed da classic "no honor among thieves" mindset, and thus was going to pull da disgraceful "run off and leave him NOT holding the bag" on his unwitting accomplice!