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.