Brains can either be "head" ie oral sex or when used in reference to a car it referes to the top (convertible) or lack there of. For example, when someone "buys an old school and blows off the brains", they are simply buying an old classic car and then getting it custominzed to have no roof or to be convertible ! The other variant of this is when you say your car has the titties out = topless
I am so sick and tired to these little white kids listening to hiphop and them having no idea of the slang, but they think they do ! that shit is so fuckin funny and pathetic !