Measuring the diameter or thickness of something using calipers, which are a tool with two legs that can measure distances.
From calipers (the measuring tool) + -ing. Calipers comes from the same root as 'caliber,' possibly from French calibre, referring to the bore of a cannon, then generalized to any measuring instrument.
Calipers were named after cannon bores because gunsmiths needed extreme precision—one millimeter off and your cannon exploded. That's why the tool became the symbol of exact measurement.
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