I think the title of this peeve says it all. In fact, I'm not sure if I can go much deeper than that, but I will put in my greatest effort to try to describe in detail what I mean.
Let me start by saying - I am no genius. But, I would like to immediately counter that with - I am no idiot, either. My grandma used to say, "You can call me ugly. You can call me stupid. But, just don't call me late to dinner." I concur, but I would like to alter this bit of wisdom to say, "Don't call me stupid." Or, at least don't treat me like I'm stupid.
You know you're being treated like a jerk when someone begins to speak in a condescending tone (this can be done just as easily in an e-mail or other form of communication.) Alerts that it is happening: meticulous, slow talking to the point of the ridiculous, tilting of the head in one direction, putting a question mark at the end of every sentence.
I used to work with a girl who always smelled a little like cheese, had a suspiciously invisible "fiance", and claimed to never pass gas unless she was seated on a toilet (something about particulates - but trust me, she didn't look that particular). Anyway, because she was on the job before my arrival, she liked to drop knowledge that you may have picked up in the womb and quickly follow it up with a question of concern about whether you could have possibly understood. Oh, I understood. And I wonder if she ever understood that I wanted to crush her like a boiled egg.