From the cult classic "Kids" -a saying of the Casper character.
Can be used to describe an array of situations ranging from "What were you thinking?" to "Damn" and including "That was unbelievable" depending on expression and intonation.
"Stupes, he deserved that shit."
"Stupes, you crashed my car into a cop!"
"Stiuuupes (read-you're a dumbass)"