It stands for "
Bad Old Noona." A Bad Old Noona is an older woman still following the young pop scene (usually Korean) as a devoted & crazy
fangirl, inappropriately attracted to insanely young pop stars (maknae
like Taemin of SHINee or Seungri of BigBang).
I
don't want to turn out to be a BON when I grow up! This
fangirl thing -- waiting all night for tickets, writing love
letters, sending him socks -- is exhausting.