'flaps' spelt backwards. It's usually used in conjunction with tnuc or Tnuc, ie someone is a 'spalf tnuc'. It's just like calling someone an idiot, i guess. No specific meaning, it's just when you want to confuse someone around you, or insult someone in front of others without them knowing.
Sam and me made this up in our economics class, after discussing 'sinep', then 'tnuc', and then 'spalf'.