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.
Its nothing bad. It means good and it is a slang term used by people. It can be used in any situation and the person above this is an idiot. It is positive and means good in French thats where it is derived from.