1) Phrase used by embittered old bastards who find that a band they liked became popular, so therefore they must suck now.
2) When a band changes style and then appeal more to
the mainstream than to it's original fans. Often a band did actually become crap when it once was good: e.g. Incubus,
Green Day. However, many bands who 'sold out' were already shit to begin with: e.g.
Good Charlotte.