The word that you use when you literally cannot comprehend the amount of stupid that a person is. They are too dumb and so you have to resort to calling someone an idiot. It's that simple.
Or
It can be about when you get something wrong and realise it, or do a mistake.
Guy 1 - See that guy over there.
Guy 2 - Yeah? What about him?
GUy 2 - He is a straight up idiot! He added a 'd' in orange.
or
Girl 1 - Omg I dropped my phone, I'm such an idiot