The phrase "let blood" is a medical term that refers to the therapeutic removal of blood from the body. This practice was common in the past as a treatment for various diseases, but it is not often used in modern medicine. Synonyms for "let blood" include "bloodletting," "bleeding," and "phlebotomy." Bloodletting was believed to balance the four humors of the body, and it was used to treat a variety of conditions, from infections to mental illness. While modern medicine has found more effective treatments for these conditions, understanding the history of bloodletting can provide insight into the evolution of medical practices and ideas about health and illness.