In medieval times, people thought bodily fluids, such as blood, controlled your emotions.
It’s December. It’s cold outside. But that doesn’t mean you’re walking around in cold blood … unless you’re also ruthless.
What does "in cold blood" mean?
This phrase describes a cruel deed done with deliberation and without mercy. A murder performed by an emotionless killer, for example.
The phrase arose from the medieval idea that blood is the seat of all emotion. Back in the day, if you got angry or passionate, your blood was thought to heat up. Even boil! As in, “I was so mad, my blood was boiling.”
In the same vein (no pun intended), medieval folk thought that when you calmed down, your blood cooled.
Thus, if you did something in cold blood, you did something unnatural. You did something violent or passionate—but without your blood heating up. That was clearly a sign of your, well, cold-heartedness.
In the book, Capote recounts the 1959 murder of a family of four on an isolated farm in Kansas. When interviewed after the crime, the killers seemed to show little emotion or regret about their actions, coolly describing how they shot the family members and left the house with a take of $40.
From what we can tell, they acted in cold blood.
In cold blood: Use this phrase to describe a dirty deed done coolly and without emotion.
Ammer, Christine. In cold blood. American Heritage Dictionary of Idioms, 2nd ed. Houghton Mifflin Harcourt, 2013.
Ayto, John. Cold, in cold blood. Oxford Dictionary of English Idioms, 3rd ed. Oxford University Press, 2010.
Oxford English Dictionary, online edition. Oxford University Press. http://bit.ly/1O5zDP0 (subscription required, accessed December 1, 2015).