One who engages in thoughtless, sometimes referred to as "stupid" actions or outbursts in speech. One can be classified a "box" if that individual frequently demonstrates careless behaviour and thoughtless releasing of verbal diarrehha, or sentences that have no relevance to a topic. Actions that are inexcusable or demonstrate impotence can be solid judgement in deeming someone a box.
Alex drank a 26 and pissed himself at a formal event, a box indeed.
Jason trying to impress his buddy, drives over a curb and loses 6 points, what a box!