Completely dry, lacking any moisture whatsoever.
From 'bone' + 'dry,' a hyperbolic compound that emerged around the 1600s. Bones, being the driest part of a dead animal, became the standard for extreme dryness in comparison.
This phrase became so popular that it's one of the few 'bone' compounds that actually refers to something completely unrelated to actual bones—it's pure metaphor that stuck around for 400 years!
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