No u: often used by a person who doesn't want to have a social interaction, with a mind that can't elaborate a complete sentence informing the other person that he doesn't want to talk, repeating "no u" for every sentence he hear, annoying every person around him.
Examples:
"Person 1: Do you want to play with me?
Person 2: no u"
"Manager: i'm going to fire you, for list of reasons
Person: no u"
Universal counterargument to any form of human insult, with the possible exception of ur dadlesbian. It is known to be tougher than Kevlar, and has catastrophic effects on the assailant.
Dick: You're fucking bieberdickass snowflake cock-juggling thunder cunt.
Bob: no u
Dick: *dies in hell for the rest of eternity*