Eyebrow: from face to roof
On your face it's hair; on a roof it's a curved window that looks like it belongs somewhere else.
An eyebrow on a face is a curved band of hair above the eye. It shields and expresses. An eyebrow on a building is something altogether different: a small dormer whose roof curves upward from the vertical wall face, then reverses direction to meet the horizontal roofline. The shape, at a glance, does resemble the facial feature, hence the name.
In construction, an eyebrow dormer is both practical and ornamental. It admits light and air to the space below the main roof while occupying minimal footprint. The curved line is signature: the roof springs from the wall in an arch, rises, and then bends back down to kiss the main roof profile smoothly. This reverse curve distinguishes an eyebrow from other dormer types, which tend to have gabled or shed roofs that meet the main slope at sharper angles. Eyebrows are small, typically low in profile, and often found in rows on long roof planes in Georgian and Victorian houses.
The metaphor is pure geometry. Both the facial eyebrow and the roof eyebrow are curved lines that frame and define an opening or feature. Both sit above something (eye socket, attic space) and both invite the eye upward. The name transferred because the shape, at a distance, reads the same way: a gentle arc that seems to express, in built form, the same subtle character the real eyebrow conveys on a face.
A different sense of eyebrow appears in textile manufacturing, where eyebrowing is the unwanted buildup of short fibers and waste on a spinning frame. The term is less common and the connection to the building sense is unclear, though the visual metaphor may lie in how the accumulated fiber fuzz resembles a bristly or unkempt line.
The dictionary entry
eyebrow
(noun, construction)
A dormer, usually of small size, whose roof line over the upright face is typically an arched curve, turning into a reverse curve to meet the horizontal line at either end.