An extremely small distance or margin; a hair's width, used to mean 'very close' or 'narrowly missed'.
From 'hair's' (genitive of hair) + 'breadth' (width), dating to Middle English. The phrase is literal—comparing the tiniest measurable distance to the width of a single hair.
Before modern measurement tools, people used body parts as reference—a 'hair's breadth' was actually useful because everyone understood roughly how thin a hair was, making it a universal tiny measurement!
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