A deep ditch, moat, or trench dug around a castle or fortification for defense, or in anatomy, a shallow depression or pit-like cavity in bone or tissue.
From Latin fossa meaning 'ditch' or 'pit,' derived from fodere 'to dig.' The word traveled through French into Middle English and retained its architectural and anatomical meanings.
Medieval castle designers used fossae not just as barriers but as water features that reflected light—making walls appear taller and disorienting attackers. Your own body has dozens of fossae, including ones in your skull that cradle your brain!
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