When a person displays that they are better than their friends, or treats their friends indifferently, especially where there has been a relationship of good standing previously; hence the term 'new'. i.e. without any history with you.
Why Jamie acting brand new like he don't know nobody? Just be cause he got a car don't mean shit, we practically grew up in the same house and yesterday he said he couldn't come over my house because it has a garage. I was like, well what about all those other 1000's of times.
acting stuck up, funny, diffrent towards your peeps,
shae been acting all brand new since she hooked up wit dat white boi.
Camon be acting brand new now that he got some new kicks.