Used when someone has done something so dumb, you can't decide whether to call them plain stupid or a darn idiot (hence the combination of the two into stupidiot). This insult is also a time saver, allowing you to give two insults for the price of one.
Marc: "I can't believe you got arrested for trying to sell weed to a cop! Jill, you are the biggest stupidiot I have ever seen!"
Lucy: "I can't believe Daren misspelled the word grapes on his project. He left off the E!"
Greg: "I know, what a stupidiot!"