Quick grammar question

OK, this isn't grammar related, but why do so many people here misuse the word "ignorant"? It seems to be used as a replacement for obnoxious / unpleasant / selfish / thoughtless / ill-mannered, etc. It is quite possible for someone to be highly educated, very knowledgeable (i.e. far from ignorant), and still act like a prat.
 

Presumably it's used in the sense of being ignorant of the norms of good manners and civilised behaviour. There's a lot of it about . . . .