Crazily stupid or idiotic, insane. Used to describe a person entirely lacking in common sense.
crazy, stupid, retarded, insane, half-baked, witless, a few crayons short of a full 52
That guy is boxcars, he just dropped his ipod in the toilet.
A bet that pays off big or just something that is awesome. Comes from the racetrack, where a winning bet on a big ticket pays so many zeros that it looks like the dots on double sixes ("boxcars" in dice). Can be used in combination with ship it.
Man, I hit that moneyline parlay and it paid boxcars.
Dude, did you see that girl's ass? It was boxcars.
Guy 1: We just scored an 8th and it's nuggy.
Guy 2: Boxcars.
Guy 1: That slampig wants to f you later
Guy 2: Boxcars, ship it.