Usually used when irritated at one's incompetence, or when embarrassed that you know someone
Also used to put down idiots or people that epically fail.
Also used in acts of sympathy for the retard that almost did, but you saved them by indicating that they Just Shouldn't.
Also said to let one know that they Shouldn't Have just in case they ever try to again.
Cool cat: Just Don't man Just Don't
Idiot: Knock Knock
Cool cat: WTF I just said Just Don't man.
Idiot #2: Who's there?
Cool cat: Why are people like you alive?
Stooge: What did the guy say when he walked into a bar?
Stooge: He said "OW" get it he said ow because he hit his head on a bar because he walked into a bar like a metal bar and not the type I ever go to because nobody likes me.
Smart Person: Just Don't
Shithead (Shi-theed): Hey guys I smoke marijuana all the time. Doesn't that make me cool?
Reasonable Individual: Just Don't (PSH)(sound)