A large railroad freight car with high sides and a roof, used to carry cargo; also slang for rolling double sixes in dice.
From 'box' (from Old Norse) combined with 'car' (from Latin 'carrus'). The term became common in American English in the 19th century with railroad expansion.
The term 'boxcar' for double sixes in dice comes from the visual resemblance—two sixes look like the boxy ends of a railroad boxcar when you're looking at the dots on the dice, showing how American slang often references the most visible technology of the era.
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