An extreme fan of a football team, in this case, the 49ers. The difference between a Niner's bitch and a Niner's fan is that a Niner's bitch thinks his team is a lot better than it is, while a Niner's fan accepts that the Niner's do in fact, suck dick.
Kevin: That faggot Parsa is such a niner's bitch.
Michael: Seriously, I can't believe he bet you $20 the Niner's would make the playoffs.
Parsa: THE NINER'S ARE GONNA GO 13-3 THIS YEAR AND YOURE GONNA OWE ME A FRANK GORE JERSEY YYOU FUCKING EAGLES BITTTCHH.