Past tense of countersink; to drill or bore a conical depression in a surface to allow a fastener to sit flush with the surface, or to submerge something beneath the surface.
From counter- + sank (past tense of sink). Sink comes from Old English sincan meaning to descend. Countersink emerged as a mechanical term in the 18th century with the development of precision manufacturing.
Countersunk screws are why old buildings look beautiful—the screw head sits perfectly flush with the surface instead of sticking out. It's a tiny detail that separates craftsmanship from carelessness.
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