cold-blooded (adj.)

also coldblooded; 1590s, of persons, "without emotion, wanting usual sympathies, unfeeling;" of actions, from 1828. The phrase refers to the notion in old medicine that blood temperature rose with excitement. In the literal sense, of reptiles, etc., "having blood very little different in temperature from the surrounding environment," from c. 1600. From cold (adj.) + blood (n.). Related: Cold-bloodedly; cold-bloodedness.