The only people who say Shut up are people who can't make good insults so use the same one over, and over.
Josh: Hey how are you?
Shawn: Shut up
Josh: Shut up you big potato looking self how about you get off of the internet and go touch some grass with your big self.
“shut up” means “Be Quiet”, but it’s kinda the Rude version.
Person 1 : Dude, that’s gay!
Person 2 : Shut up man.
Person 1 : gee ok, you don’t have to be mean about it. Was it that Offensive?
Person 2 : You’re literally gay bro.
Person 1 : Shut up! You can’t be talking your also gay!!