What Does 'In Cold Blood' Mean?

'In cold blood' means to do or execute a cruel act without any feeling of compassion. It refers to committing a heinous or gory deed without any semblance of guilt or mercy, utterly ruthless. For example, "he was killed in cold blood by the Mafia". An act done in cold blood refers to an act that is callous and cold-hearted. A cold blooded person is usually cold and unemotional. He is generally not affected by emotion, neither does he show any.

This is actually a metaphorical usage of the word cold-blooded derived from cold-blooded animals, meaning a person who basically does not possess warmth.