A fan of and/or believer in Justin Bieber. In case it wasn't obvious, it's a combination of believer and Bieber.
When Neil Diamond's and Smash Mouth's hit song "I'm a Believer" was remade to "I'm a Belieber", all JB's fans similtaneously Biebergasm
ed and then procceded to Bieberblast
the song on iTunes.