Pretend you’re a 3 year old and a meanie head calls you a name, you respond with “nuh uh, you are!”. It has been shortened to “No u” to reflect the continuing degradation of our population.

Wtf are you high? "No u" is one of the basic human responses: "I know you are but what am I", "right back atcha", "no you're a goddamn bastard!" The phrasing changes but the sentiment is the same.

It’s just a dumbed down way of saying “no, I’m not the one that needs to reflect on reality: you are.”