joudan ja nai you, though i can only understand a little japanese, i think it means, "don't kid around" or "it's not a joke" something like that.. ^^ it depends on how it was said anyway. ^^

Jodan=Joke /Ja(dewa)=is/ Nai=Not so, jodan janaiyoo= I'm not kidding, Don't be kidding, I'm serious.

don't be kidding jyoudan = joke