It's what you say when you don't know what else to say to someone, and you're thinking "Wtf", but they're your friend and you don't really want to sound mean. If used with authority figures it can also mean "Fuck you".
The response you get when you say the most vile, absurd, hateful, harmful, horrible, terrible, disgusting, despicable, delusional, terrifying, deliberate, unfathomably evil words to ever come out of your mouth.
“hey girl are you a pumpkin cause I want to slice off the top of your head, scoop out your insides and cut your face up and hang it outside my house”
“Oh.”