Someone who talks down to or belittles someone else or basically treats that person as if they are of a lower life or class. This is best exemplified (and named after) in the popular show "The Office" when Michael or Dwight do something silly and Jim gets that look on his face like Michael and Dwight are children. The person who acts like he is in someway superior to other people is the "jim."
Wow, that kid's being such a jim.
I just told a joke but my friend didn't like it and was being a jim about it.