Drilled or enlarged with a special enlargement at one end, typically to recess a bolt head or screw head flush with a surface.
From counter- (opposite direction) + bore (to drill). A technical term from machining and engineering, describing a specific drilling technique developed in industrial manufacturing.
Counterboring is why boat hulls and aircraft fuselages are so smooth—engineers use it to make fasteners completely invisible, reducing drag and improving aerodynamics.
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