Like, so obnoxious that other children (who, by nature, can be a little obnoxious) notice it.
Imagine how much it would take to ruin a magical experience like that of The Polar Express.
He gets a little better by the end of the movie, and he may grow up to be a perfectly normal dude, but still, I’m not willing to take the chance and let this dweeb ruin my holiday.
You’re headed to the North Pole to meet Santa Claus, and you have this loudmouth kid going on about how everything works, and just being obnoxious.