I would define the term as someone who isn't stupid but treats others in an arrogant or disrespectful way. A good synonym would be jerk, because he doesn't care about other people. He isn't slow, he chooses to be an ignorant person. I won't elaborate on the origin of this term, because it is inappropriate for this forum . I should have said it before, this term is definitely offensive, not slightly offensive.

It's really never appropriate to call someone that, as it is a very offensive term and is likely start a fight. Usually it is what someone calls someone else who they dislike when that person is not present to hear it, such as when you're talking about a person behind their back with your friends, or when you're in traffic and someone upsets you and you say it to yourself. The word itself means something you use to clean feminine areas (I won't go into more details than that, because like Scott said, it's inappropriate), but mostly it's used in the same way as "jerk" or "tool". Basically, a person who is rude, or acts macho or overly-confident in order to make themselves appear more important than they really are.

It's the American equivalent of "wanker."