When we refer to someone as arrogant, we nearly always think of arrogant as obnoxious and impossible. There can be a thin line between confidence and arrogance, but confidence can be feigned, and often arrogance is the result of an inconfident person who has tried too hard to over-compensate.
Many people on the autistic spectrum will often appear arrogant, without it being their fault at all. And depending on their level of perception, they will range from totally indifferent to absolutely mortified once they realize they have been perceived to be arrogant.
One interesting thing about arrogance is that as a fault, it is never in isolation. A person with arrogance will most likely have many intolerable flaws.
Think of someone you know who you consider to be arrogant. What are their other crimes against humanity? And are those crimes listed in the Symptoms page on this site? When I try this exercise, all the flaws in the person (or variations) are listed. For this reason I have come to the conclusion that all people who come across as arrogant to others on a regular basis (whether knowingly or not), are most likely somewhere on the Autistic Spectrum.