The term 'sellout' has been around for a few
generations. Often used as a negative term used to describe somebody that conforms to a
particular way of doing something such a dress, vernacular, or type of work.
Recently it has been used in the indy music scene to a large extent since the explosion of ska, punk, hardcore, and indy rock.
One earns the label of sellout by alienating their core fans that have supported the band by changing their style to be more appealing to a broader audience, often become what they were rebeling against in the very beginning. While some may call this "evolution of the artist/music" one has to remember that evolution is a gradual change and not sudden and drastic, the losing of edge, and doesn't accompany a bigger record label.
While many bands out there refuse to be
labeled as sellouts, they are often
putting out records and being quoted as saying "This is more true to our core sound, where we started".