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"