The word "铁石心肠" is formed by directly combining four characters: "铁" meaning iron, "石" meaning stone, "心" meaning heart, and "肠" meaning intestines; here, "心肠" together functions as a metaphorical term for one's emotional core, so the compound literally denotes "iron-stone heart-intestines" to convey the idea of a heart as unyielding as metal and rock, thus describing a person who is unfeeling or hardhearted.