Translation:It is not like you to be so impossible.
If somebody is being incredibly stubborn and it's a little out of character for them you could say, 'you're being impossible!'. Or if you are trying to have a debate/discussion with someone who just refuses to listen to any reasonable arguments you could use the same phrase :)
If someone is being 'impossible' it means they are being difficult/impossible to deal with.
If it is 'not like them' that means that they are behaving in an unusual way.
So this sentence means "you are being difficult to deal with and that is unusual."
It's not a common phrase but you might use it if someone you normally get on with is being frustrating. I can imagine a teacher saying it to a normally good pupil who is having a bad day, for example.