Someone who is very dumb and/or has made a very dumb decision. Usually an exaggerated expression.

A large TV screen that messes up, usally said during schooling or professional meetings to add a bit of humor.

A person who is foolish and brings rejection and embarrassment upon themselves in full view of their social group or the general public. This phrase was first coined by a nerd in Australia, and is a conjunctive of dumb and jumbotron.