Poseur is the correct version of the more common "poser". The term is used for 2 reasons-
A) As an insult, used by one who is insecure and attempts to enhance their image by making people believe they have the authority to call someone else a poseur,
B) One who groups him or her self in a particular social clique in order to appear trendy or "edgy" but doesn't believe in the principals held by said clique and is only leeching onto it in order to avoid the social ridicule of not being part of a clique. A poseur will probably change their "style" once every 6 months, possibly more.
The common thread is insecurity- the insecurity to be yourself and be happy with it. Poseurs are the kinds of people who only appear "cool" to people who are just as insecure as they are. But no one ever found success lying to themselves for acceptance.
"Those mall punks are such poseurs, they have anarchy signs written all over their All-Stars, but nota single one of the retarded 8 year olds knows what it is, or that the sweatshopped armbands they all bought at Hot Topic support the system they claim to be rebelling against. Now lets go home and listen to some Yellowcard and cry so we can be hardcore," says the pussy emo kid with the rich parents and cute girlfriend, who has nothing to complain about, before I punch him.