The breadth of a hair, taken as the type of an indefinitely minute space or line, literal or figurative. See hairbreadth.(noun)
The Century Dictionary (Public Domain)
Use "hair's-breadth" in a sentence
"Halfway through, the wind hauled to the north'ard of northwest, the glass dropped to 28.88, and he turned and ran before a gale of cyclonic fury, missing, by a hair's-breadth, piling up the Mary Rogers on the black-toothed rocks."
"Delicate to a hair's-breadth, hand and eye true and steady as steel."
"The words were either false when spoken or else so misleading as to evade falsehood by a hair's-breadth equivocation."