A recreational leaning drink- an alternative to illegal drugs in pure form- that is common in parties and originated in the south.
It generally consists of around two fluid ounces of promethazine/codeine cough syrup, 8 or more ounces of lemon-lime soda (Sprite, Sierra Mist, 7up, Mountain Dew, Fresca, etc), and jolly ranchers for flavor. It is called 'purple drank' for a) the purple color that the dye in the syrup gives it, and b) the southern pronunciation of the word 'drink'.
It was created around the time when chopped & screwed rap music hit the scene, where the drink went well with the music considering it induced a similar 'slow' and 'leaning' feeling in the consumer. The beverage until recently, however, did not become popular on a wider scale, and originally was almost completely limited and known (for that matter) to the southern regions of the USA.
Purple drank has more street names, the most popular being Lean and Sizzurp.
"Mike Jones keeps purple drank in his cup."
"Dude, that drank got me leanin' last night."
"That drank tastes like shit, but feels like sex."
Prices shown in USD.