Definition of hairline
1
: a very slender line: such as
a
: a tiny line or crack on a surface
a hairline bone fracture
b
: a fine line connecting thicker strokes in a printed letter
2
: hairbreadth
3a
: a textile design consisting of lengthwise or crosswise lines usually one thread wide
b
: a fabric with such a design
4a
: the outline of scalp hair especially on the forehead
b
: the way the hair frames the face
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Merriam-Webster unabridged
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Time Traveler for hairline
The first known use of hairline was in 1846
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