Used as a rebuttal.
Originated on the glorious 4chan.
When opposing a comment or post : No U!, means "No, you are!"

E.g.
Pedofan : You are a faggot Moot!
Moot : NO U!

The universal rebuttal to all arguments.

The end all to any argument. Used when one either has no intelligent argument left, wants to try and use "no u" as an intelligent argument, or want to piss someone off. Most effective after one has been insulted.

The ultimate trump on the Interwebz. Ends all arguments and debates.

Similar to the religious argument, "you have no faith."

Saying NO U is a way of pointing out that the person's comments apply more to himself and is therefore an idiot and a hypocrite. It is considered to be the ultimate trump to any debate or drama on the Internet.

This meme originated on the Something Awful forums where epic flame wars would often erupt. Eventually the arguments became so illogical and the insults so repetitive that often it needed to be pointed out that the OP was being a hypocrite.

NO U is originated back to simpler days when we as children would engage in what were to be the precursors to forum wars. Arguments over common issues would devolve into anarchy as neither party was willing to concede their point. This inevitably led to attacks on character which subsequently devolved into childish name calling which of course would usually finish with NO YOU. A major problem is that it can start an endless loop of NO U. These are known "No U Wars."

As long as there are forums where any asshat with an opinion can post, NO U will always have a home. Short of ignoring idiotic comments, this is a simple and effective way of pointing out that the OP’s comments are in fact more aptly applied to himself than his subject.

"In actuality what has been stated applies more directly to the former poster than to myself."

Generic answer to almost all statements. Often used in a random sense.