Acronym for Boyfriend Freak Out Syndrome.
When you like someone, get to know them then freakout, run away (or hide yourself in the toilet), and/or do something stupid to sabotage your chances with a guy you like. Usually at a point when it is most important in getting a date, etc.
Friend: Why did you run into the toilet just as that guy was going to talk to you?
You: I dont know I guess BFOS got ahold of me.