Callus
A callus was an area of thickened skin developed by repeated wear. Metaphorically, it referred to areas of emotional hardening.

In 2364, Doctor Beverly Crusher, concerned about those on Styris IV who had contracted Anchilles fever, wondered aloud, "Damn. Where are the calluses we doctors are supposed to grow over our feelings?" Jean-Luc Picard then opined that perhaps good doctors didn't get them. ()

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Medical conditions
Psychology