A word derived from hearing pikeys talk to each other when saying "Yeah mate", in a particularly common southern English accent which ends up sounding like "Yeah bart" when muffled or when said while under the influence of alcohol/drugs.
An offensive term used to describe pikeys or scallys.
It can also be a sarcastically affectionate term used to talk to friends by replacing "mate" with "bart", when greeting them.
"Yeah bart hows it hanging?"
("Hello my friend/mate how is life treating you?")
"Look here come some barts"
("Some pikeys are coming towards us)