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.