ne seri

Interjection

 * 1)  an expression of amazement, shock or disbelief
 * 2)  an expression said to someone when they are talking in a long-winded way, or lying in general
 * 3)  no shit; a sarcastic response to express the view that someone is stating something very obvious.
 * 1)  no shit; a sarcastic response to express the view that someone is stating something very obvious.

Usage notes
As for the third sense, it is more commonly used in everyday conversations (spoken in a sarcastic tone, obviously), and not on the internet, due to people possibly not understanding in what expression you're saying "ne seri" in, as it's connotation has been mostly twisted to the second sense. More commonly, on the internet, people say "nemoj zajebavati", or add any other sarcastic prefix (such as "ma").