humor noun

  1. The quality that makes something laughable or amusing; funniness: could not see the humor of the situation.
  2. That which is intended to induce laughter or amusement: a writer skilled at crafting humor.
  3. The ability to perceive, enjoy, or express what is amusing, comical, incongruous, or absurd. See Synonyms at wit1.
  4. One of the four fluids of the body, blood, phlegm, choler, and black bile, whose relative proportions were thought in ancient and medieval physiology to determine a person’s disposition and general health.

That’s not the complete definition from the American Heritage Dictionary, but it’s enough for now.

Modern English humor derives from Middle English, where humor largely referred to a bodily fluid; Middle English borrowed Old French umor, which is largely a borrowing from from Latin ūmor, hūmor, meaning “fluid.” In earlier eras, our bodies were thought to be affected by the balance of four humors or fluids; blood, bile, phlegm and choler. These fluids, and their balance, determined which of four temperaments (sanguine, melancholic, phlegmatic, or choleric), were dominant in an individual. Each of the humors was also associated with one of the four elements.

The basic theory behind the humors was developed by Hippocrates (460-370 BC) who theorized that human psychology, or in his terms, moods and emotions with their associated behaviors, were caused by body fluids or humors. The specific fluids were blood, yellow bile, black bile, and phlegm. Hippocrates’ ideas were central to early medicine and were used for treatments of disease. Subsequently, Galen (AD 131-200) wrote a treatise on the effects of the humors in combination to develop temperaments in De temperamentis. The root behind temper and temperament means “mix” and Galen attempted to determine the specific physiological causes for various human psychological states Galen associated the humors with the four elements, and described the resulting states as combinations of hot/cold and dry/wet.

Blood Sanguine Happy, extroverted, emotional, and talkative.
Yellow bile Choleric Ambitious, energetic, active, and often dominant; typically associated with military leaders.
Black Bile Melancholic Introverted, self-involved, forgetful, worries and stresses. Often associated with poets and writers.
Phlegm Phelegmatic Self-content, often kind, consistent, rule-based, fond of stability and tradition, reluctant to change, loyal.