A person who was an asshole to you, but the rest of the world either doesn't care or doesn't know about it, or denies the fact that the person did it. It's usually a person who is a popular or at least a well-liked person taking advantage of their social status and using other people lower on the totem pole to get what they want. You can find an incognito asshole or two at your local middle or high school.
"Today, I won $50 on the lottery. On the subway home, I checked my pocket to see if the money was still there. A very professional man in a suit yelled, "Hey, that's mine!" I got several dirty glares. I'm such a pathetic wimp that I gave him the money. FML" is a great example of an incognito asshole.
Another example would be:
Person 1: Ricky, that scumbag, stole my phone!
Person 2: What are you talking about? That's Ricky's new cell phone.
Person 1: I don't care what you say, Ricky's an incognito asshole!