In cold blood

Many expressions in English derive from the perceived temperature of blood. We talk of doing something in hot blood or in the heat of passion. Sometimes our blood boils at other times it runs cold. Sometimes it even curdles. We describe people as hot-blooded or cold-blooded. Not surprisingly, to do something in cold blood has come to mean in a detached, callous and unfeeling way, the very opposite of doing something in hot blood, which is more forgivable. In cold blood dates from the late 1400s. Shakespeare used the expression later in Henry IV Part II, Act IV, Scene III, “The cold blood he did naturally inherit of his father.” He also used hot-blooded and cold-blooded: King Lear Act II, Scene IV, “The hot-blooded France that dowerless took our youngest born” and in King John Act III, Scene I, “Thou cold-blooded slave.”