When you sprinkle Opium on top of your Marijuana(Dope).
"Dude, this bud tastes funny."
"Yeah I know man, it's dopium, I sprinkled some opium on it.
1.) A mixture of Opium and Weed.
2.) Something that is awesome. Something that stimulates the senses.
"Dude, this pizza is utmost Dopium."