An ass that you cannot see. According to the person with this ass, it's apparently there, but not visible to the naked eye. This is just an excuse for a woman with a really flat ass.
Finneh: I caught some bitch looking at my ass a few days ago.
STEFFFF: maybe it was my ass
Finneh: What ass? OBRN!
STEFFFF: its there. =(
Finneh: So apparently you have a ninja ass.