To be incensed is to be beyond mad. It's how you feel when fighting with your cell phone company's customer service representative whose only response is to quote company policy verbatim from a script.
The Latin root of the word incense is incendere, meaning "to set on fire." Yes, it's the same root for the incense sticks burned in dorm rooms and churches. For this use, however, think instead about burning tempers instead of burning sticks.