The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018
Humours: From the Latin for “moisture,” a physiological theory, espoused in ancient times, the Medieval Period, and the Renaissance, that held that the relative amounts of or balance between the four main fluids (humours) of the body — blood, phlegm, yellow bile, and black bile — determined an individual’s state of health and even general personality. The four humours were also associated with what were then considered to be the four elements: blood with air (hot and moist), phlegm with water (cold and moist), yellow bile with fire (hot and dry), and black bile with earth (cold and dry).
Adherents of the theory of humours believed that the humours emitted vapors that rose to the brain, thus affecting both behavior and health. As long as the humours were in balance, the individual supposedly exhibited a perfect temperament and no illness, but an imbalance affected behavior in a very specific way. That is, an excess of blood produced a sanguine (happy) personality, phlegm a phlegmatic (cowardly, passive) one; yellow bile made the personality choleric (argumentative, stubborn), and black bile made it bilious (melancholy). Just as an imbalance produced a distinct behavioral effect, so would it produce illness and disease.
This theory was so widely accepted that it made its way into popular culture and literature. Individuals and literary characters came to be classified according to their humour, and the word humour itself came to signify a variety of things, from disposition or mood to peculiarity or affectation, particularly in Elizabethan times. Many works of literature even relied on this theory for characterization and to provide convincing motivation for characters’ actions.