Top Definition
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 much attention.
by Electric Sheep March 15, 2009
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