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.