White cup is a term sometimes used to replace "Purple Drank or Lean." A drink popular in the south that consists of Codine cough syrup and Promethazine mixed with a soda or fruit juice. White Cup can also be used in reference to drinking "Purple Drank or Lean."
"What be your favorite drink crip." Shit you know I drink that White Cup boy!
"Yo crip, what you got in dat cup you be sippin on?" Nigga can't you see dat is a White Cup!?