Describing a person who commits violent acts without feeling emotion or remorse, or describing animals like reptiles that regulate body temperature through their environment.
Compound of 'cold,' 'blood,' and 'ed.' Used both literally for reptiles and metaphorically for emotionless humans. The literal sense predates the figurative sense meaning 'cruel without emotion.'
It's confusing because the word means two completely different things: reptiles are coldblooded because they can't generate body heat (they're actually wonderful!), but calling a human coldblooded is one of the strongest insults in English!
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