Acting brandnew is when you screw up something that you should full well know how to do. It can also mean being afraid to do something when all of your other friends are up for the challenge.
" Why you being a bitch? You actin' real Brandnew right now!Or, "How the hell you spilled my drink? Damn, you Brandnew".
Brandnew is when someone acts as if they are the shit and better than someone else. It could also be used to described a person who used to act one way but now acts totally differnt.
" I don't know why that bitch wanna act all brandnew now that she got a man."