The quality or state of being shaped like a box; a square or rectangular appearance.
From 'boxy' plus the noun-forming suffix '-ness' (from Old English), which converts adjectives into abstract nouns representing qualities or states.
The transition from 'boxy' to 'boxiness' shows how English constantly creates abstract nouns to discuss qualities—we can describe a car's boxiness in a way we couldn't easily do with 'boxy' alone.
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