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.
The HUGE display screen typically found at sporting events. It shows game/player stats, scores, replays, advertisements, etc to the crowd.
Jumbotron is actually a Sony trademark name, but the word is used to refer generically to any gigantastic LED screens like it.
At a basketball court, the jumbotron might be about 11 feet wide, but in a football stadium, it'll probably be more like 33 feet.