The chalky residue or ash left behind after something burns, or the heel bone in anatomy.
From Latin 'calx' meaning heel or limestone, derived from the root referring to hard mineral deposits; evolved to describe both the physical heel bone and the burnt remains of calcination in chemistry.
Roman soldiers marched so much that 'calx' (heel) became their word for a key body part, and chemists borrowed the same word for ash because both are hard, mineral-like substances—language recycling at its finest!
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