When you take a dose of Ecstasy (MDMA) accompanied by a dose of Acid (LSD) and you get the good feeling from the Ecstasy, but also more vivid hallucinations from the Acid by mixing the two.
> "Hey dude, I've still got a few hits of that acid I got from John last week. Why don't you go get a couple of pills and we'll go candy flippin' tonight?"