When someone states that they are hanging out with a friend that is fake or made up. If a person is trying to hide something (i.e drugs, fat/ugly girl or guy)then they make plans with their placebro to keep friends from getting suspicious.
Bobby has been hangin out with this guy Jim every Friday night. Not one of his friends have ever met him. I'm starting to think Bobbys smoking crack and Jim is a "Placebro (pla- see- bro)" .