Relating to the ribs or the sides of the body near the ribs, or relating to a coast or shore.
From Latin costalis (from costa 'rib') in anatomical usage; separately, from Old French costal relating to coast/shore, creating two semantic meanings in modern English.
English sneakily inherited two different meanings of 'costal' from different sources—one from Latin doctors talking about ribs, one from French traders talking about coasts, proving how language borrows from whoever dominated each field.
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