It's basically a word used in the modern internet word. It means that if someone says that someone is "Haha you're gay" but in this occassion, they say "No u" back, which turns their words against them.
Alex: "Lmao go f**k urself"
John: "No u"
Alex: *ceases to exist*
When Your In A Argument, or In a Insult Battle (Same thing) if they speak about you or your family just say no u
Person 1 : Go Die in a hole you useless peice of shit
Person 2 : No u
Only use this phrase as a last resort option, even against the deadliest of foes. There is no coming back from this one. Once said, the damage will be reversed and your opponent's beliefs are destroyed.
A: Ur mom.
B: no u
A: *ruptures his appendix and fucking dies*
An urge to sleep with the person above you
Sam: shut up
Bob: no u
(Sam and bob fuck)
literally the most cringeworthy thing to say in 2020 and beyond
if you say this youre most likely around the age of 8-10
but if youre 15+ and say this i dont know if your parents loved you enough
sometimes used in arguments but is now called the forbidden, and you will be made fun of if you say this
person1 : "youre bad"
person2: "no u"
person1: "did you fucking seriously say that youre honestly a cringe asshat go die immediately"
person2: "no u"
person1: "your parents never loved you enough"
person2: "thats.... right"
Jo u looks like an optimised gaming monitor, made out of expired onions.