Lean is a mixed drink originating in the Southern Rap culture. It is a mixture of Promethazine and Codeine cough syrup and a soft drink such as Sprite (usually). While other soft drinks may be used, Sprite was the original.
There are some variations of Lean. For example, Promethazine and Codeine syrup is usually Purple in color. But there are other colors of syrup that work the same way. There is a golden colored syrup (hydrocodone based) and other colors as well.
Lean slows you down. It makes you feel good. It's meant to be sipped on, and it taste damn good. One of the best feelings you will ever experience. Euphoria with a hint of sedation.
Chris: Yo man want to go to a party tonight, I heard they got kegs there?
John: I can't man.
Chris: Why not?
John: I'm sipping on that lean already bro. Don't need that alcohol.