It's hydrocodine syrup, the kind they give for really bad allergies, it's use to soothe pain and as a powerful decongestant. At large doses it will mess you up.
Typically as referred to by most rappers, it's codine syrup and literally the drink purple drank, yes it exist you can get it at most gas stations now, it taste like purple koolaid except is has a very small dosage of valium in it, so small that it's legal to sell with a mix of other herbs which supposedly "slow your roll" as it's advertised. Also, purple koolaid is often used as a substitute.
Swaggajuice is also thought to be largely composed of crown royal and lil'wayne's tears.
Can also be known as Swaggadrank.
Gangsta1: "Yo man, where be all dat Swaggajuice at??"
Gangsta2: "I dunno man..... you must have drank it all!!"
Gangsta1: "Dang man, that'd go great with all my fried chicken!!"