I get more and more confused every time I hear this term...first I thought it was when you intentionally do things to get your way through manipulation. Or like if you're friendly to someone in person but talk negatively behind their back and do subtle things to 'insult' and/or upset that person and then claim to have done nothing.

Being condescending or rude seems much more like direct aggression not passive aggression....uhh either way both those behaviors are unpleasant to be on the receiving end of.