Someone or people (normally a band) that forget their morals or principles to sell records. They give off their values for money. Many people say Green Day are not sell outs because they never did anything that they said they wouldn't, but they said they were punk. Punks do not go on MTV. Punks do not go on Top of the Pops. Punks do not sign up to big record lables. Green Day, like other sell outs, rocked when they had souls.
Listen to EMI by The Sex Pistols, true punk who never sold out. Green Day probably think they are better than the Sex Pistols because they played a bigger gig, but what matters is what the true punks think.
Billie Joe: So do we play this Milton Keynes gig or not?
Tre: One moment while I do something wild and zany in an attempt to be amusing.
Mike: How much is it paying?
Billie Joe: Shit loads.
Mike: Hell yeah! Let's do it!
Billie Joe: Don't forget to never stop claiming that we're not sell outs.
Sell outs. Pure and simple.
Green Day have sold out by going on MTV, Top of the Pops etc.