铁骨 is a literary term that literally means 'iron bones' and is used metaphorically to describe a person's unyielding, resolute, or indomitable character. It often appears in phrases like 铮铮铁骨 (unyielding iron bones) to praise someone's firmness and integrity. In a more literal sense, it can refer to a dark-colored pottery body (the clay base before glazing), but this usage is rare in everyday language.