Dimness or darkness of vision; a medical condition affecting eyesight, or figuratively, any confusion or darkness of understanding.
From Latin caliga meaning 'darkness,' 'mist,' or 'dimness,' with military associations (Roman soldiers' helmets were called caligas). The term entered medical vocabulary to describe vision problems and metaphorically, mental confusion.
The Romans called their heavy military helmets 'caligas' because they create such restricted vision, so this medical term for blindness literally means 'the condition of seeing through a helmet'—what a perfect metaphor for limited perception.
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