Plural of calx; either heels of the feet, or the residual mineral matter remaining after burning or calcination.
From Latin 'calx' meaning both heel and limestone/lime. The plural form 'calces' follows Latin declension patterns and entered English through scientific and anatomical terminology.
The same word 'calces' means both the heels of your feet AND the ash-like residue left in a furnace—the Romans' poetic mind connected these different meanings through the idea of what's left behind!
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