E=MVΔCrack

It's when you smoke crack off of someones crack for energy, particularly someone of the same sex. It's a chemistry joke and a play on the equation of combustion energy E=MVΔT

Guy 1 "bro I was so tired so I had to smoke up for some power."
Guy 2 "E=MVΔCrack?"
Guy 1 "hell yeah mofo you know it."