Usually accompanied by a picture of someone pointing, NO U is slang for "No, you". The comment simply implies that a particular poster ought to apply his suggestion to himself. That is, if Anonymous says "Your statement is stupid and you're an idiot" and Anonymous replies "NO U" he is telling the first Anonymous his statement is stupid and he's an idiot. This is mostly a sarcastic way of pointing out how internet debates all too often end up. Most frequently accompanied by a picture of a yelling Asian man with his pointing hand shopped into various places. Has its origin in the depths of the Something Awful forum; similar in spirit (but not related to) "Omae mo naa!".