To "have it up" means to have drugs on you or be in possesion of drugs, which are for sale, mainly what ismarijuana. This is said when a buyer wants to buy off a dealer but before showing his loot he will ask his dealer "do you have it up?" to confirm hes got chronic on him.
custy: "you have it up?"
me: "yeah how much you need?
custy: "a half quarter"
me: "aight yo this is boom shit"
a phrase that famous YouTuber Cody Ko will say next time he sees someone bully someone else in public for not being able to game because they're a girl