Short for "powwder," in reference to cocaine. Used almost exclusively by those affiliated with Three 6 Mafia and the Hypnotize Camp Posse. While some believe the "P" used in many rap songs is referring to "pills", it is made very clear in many of Three 6 Mafia's older albums that this is not the case. See example.
"Chorus 4x Smoke, smoke, smoke dat weed
Smoke, smoke, smoke dat weed
Drank, Drank, drank that liquor
Pop them pills, snort that P"
-Juicy J "Smoke Dat Weed"