1. Substance D is a fictional addictive amphetamine-like psychoactive drug, also known as "Slow Death", "Death" and "D", that is depicted in Philip K. Dick's novel "A Scanner Darkly". It's derived from the fictional flower "Mors ontologica".
2. By extension, Substance D can refer to amphetamine (speed).
"What he sold me as 'candy' actually turned out to be 'Substance D'."
This is a more subtle way to refer to amphetamine in a conversation if you don't want to attract too muchattention.
the act of going out on holidays or the weekend and drinking to the point where you can't talk coherently and wake up hand-cuffed to a hospital bed the next morning.