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.