A horse or animal with a calm, even temperament; also, a person who commits a crime without emotion or remorse.
Compound of 'cold' and 'blood,' originally describing horses bred to be calm and steady rather than spirited. The metaphorical use to describe emotionless criminals emerged later by analogy to this horse breeding term.
Horse breeders literally created a type of horse called 'coldblood' to be calm and strong for farm work—nothing to do with actual body temperature! But then people started calling criminals 'coldblood' because they seemed equally emotionless.
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