So right off the bat that is not a word I would use to describe a person. I know that it is a low insult to everyone with any mental impairment far more than whoever is being an idot that would have been called such. 

However in my mind it is a word with a definition and there is no reason not to use it. 

What brought this to my attention was a person who interrupted me and proceeded to insult me and call me an asshole. I was just having a conversation with a friend about my motorcycle and the problems I'm having with it, perticularly the fact that I had to retard the ignition significantly to get it to start. 