An extremely small distance or margin; often used in the phrase 'by a hairbreadth' to mean narrowly or by a tiny amount.
Compound of 'hair' and 'breadth,' referring to the width of a hair as a measurement. This emerged in Middle English as a vivid way to express an impossibly small distance.
A hair's width was such a precise, tiny measurement that English speakers used it as their measuring stick for 'barely' and 'almost'—our ancestors didn't have micrometers, so they used biology.
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