Says you. Man A: You are so stupid! Man B: Says you! Normally this is a reply to hypocrisy. One person will make a statement towards another, and if the second person does not agree and thinks that the statement is more applicable for the person that said it, then "Says you!" is an immature way of saying~ "You are a hypocrite."

Says you-Oh yeah, says who-You and what army. I learn new words.Thank you Alex^-^

"That's the way the cookie crumbles!" - if something is unfair, too bad.

Are you by any chance watching/reading something with children? "Says you!" means like, you're saying that but it isn't really true; your information isn't accurate. It might be preceded by "oh yeah". Here's some more English (as spoken by children amongst themselves): "oh yeah, says who?" - who is the authority for this fact/rule. "You and what army?" - how are you going to force me to do something, or take something from me, or beat me in a game or fight (possibly, although this could be spoken kind of in jest, but not always). Anyway, this is how we talked when I was a kid, coming home all covered in mud and dirt.