The phrase "in cold blood" is an idiom which means to act in an unemotional manner, without feeling or passion. Today, it is most frequently used when referring to murders which were not emotionally motivated. The origin of the phrase dates back to the early 1600s and is based on a medieval medical belief.
Originally, this phrase was once thought to be much more literal than it is considered now. Early physicians once thought that a person's emotions and internal fluids were closely connected. Blood, for example, was thought to literally heat up when a person became angry or impassioned.
Conversely, when a person was calm, with little or no emotion, their blood was thought to be cool. So although saying someone did something "in cold blood" in the 17th century meant they were calculated and free of emotion. It was also thought that the person's blood was literally cooler than an individual who was emotional. The terms "cold blooded" and "hot blooded" are also derived from this idea.
Some linguists also think the phrase might have been originally translated directly from the French word sang-froid. Sang-froid means "calmly" or "with composure," but translates literally to "blood-cold." The French term, however, has a generally positive connotation whereas "in cold blood" is more often used negatively.
According to the Oxford English Dictionary, the first recorded use of the phrase "in cold blood," was in 1608 by an English soldier, Sir Francis Vere in his Commentaries of the Divers Pieces of Service. Vere stated that he was writing "a resolution framed in cold blood." The phrase was used in various documents over the next hundred years, appearing in literature in 1711 when Joseph Addison used the phrase in his periodical, The Spectator, to describe a murder.
Perhaps the most famous use of the term in recent history is Truman Capote's 1965 book, titled In Cold Blood. Capote and the book gained fame and infamy when Capote, a journalist, swore the book's account of events was pure fact merely told with a fictional style. Controversy broke out when several people featured in the book accused Capote of making up major scenes and misrepresenting characters. Infamous or not, the book revived a style of journalism used by Mark Twain almost a century before. Now dubbed New Journalism, Capote's fiction-writing techniques are considered essential in most journalism philosophies.