Basically humor = “liquid, moistness”, from Latin, and it most commonly referred to four bodily fluids (humors) that were thought to control human emotion and temperament. It didn’t come to refer to something funny or comedic until the 1680s. Read on if you want more info because I fell down an etymological chasm dripping with literally all of my least favorite words (phlegm, bile, moist) and now I’m feeling a tad queasy.
Humor. What a staggeringly rich and shockingly vile word.
If you’ve heard of the concept of “the four humors” in the sense of bodily fluids that control emotion, you may not be surprised to learn that the word humor comes from the Latin word umor, “bodily fluid,” which in turn came from umere, meaning “be wet, moist.” It slipped into English via the Old North French word humour, which meant both “liquid, dampness” in general and referred to the four humors.
If you’re unfamiliar with the concept of humorism/humoralism as a medical system, it basically explained human emotional and behavioral tendencies in terms of four types of liquid: blood, phlegm, choler/yellow bile, and melancholy/black bile.
The concept of humorism is thought to have originated in ancient Egyptian medicine and was systemized by ancient Greek physicians and philosophers. One leading theory suggests that the four humors were based upon the observation of blood clotting in a glass container; after sitting an hour, four different layers can be seen in the blood.
Drawn primarily from the works of Hippocrates (who is credited with identifying the four fluids/humors), humorism was also often associated with Empedocles and Aristotle’s four elements (earth, fire, air, water), and the works of Galen, who described human behavior as a balance of temperature and dryness/moistness. Each humor was also associated with the folllowing: an element, a season, an age, a temperature/level of moistness, and a bodily organ that was thought to produce each humor.
Various problematic character qualities and moods were considered to be caused by an imbalance of these liquids in your body. These imbalances also became a way to generalize differences in temperament/disposition among various demographic groups.
Because it’s interesting (and slimy), I’ve chosen to briefly look into the etymologies and effects of the different humors.
Blood
A really frickin’ old word, the English word “blood” comes from the Proto-Germanic blodam, which in turn came from the PIE root bhlo-to-, likely meaning “to swell, gush, spurt” / “that which bursts out.” (That root, interestingly, is related to the PIE bloma or “flower” which offers the same sense of blossoming outward.)
In its sense as one of the four humors, blood was associated with the element air, as well as heat and moistness, spring, and infancy. People with too much blood were thought to be “sanguine”—which literally means “bloody” but is also used to mean overly happy or prone to fall in love. Blood and the emotions associated with it were thought to be produced by the liver. The early medical practice of “bleeding” a sick person was thought to help relieve an imbalance of blood.
Phlegm
In a disgustingly Rick-and-Morty-esque turn of etymological nastiness, the word originally glopped into English in the frothy form fleem, a 14th-century word meaning “viscid (glutinous, sticky) mucus.”
Fleem came from the the Greek phlegma, which—you guessed it—was one of the four humors and meant “humor caused by heat” thanks to its etymological parent phlegein, “to burn.”
Somewhat counterintuitively given that “burn” connection, the phlegmatic humor was associated with coldness and moistness, winter, water, and maturity. (The heat vs. cold inconsistency may be a result of changes in humorist philosophy over time.) Phlegm was believed to originate in the brain and/or lungs, and an imbalance was thought to cause apathy (hence the Modern English definition of phlegmatic as “cool, calm, self-possessed” or “cold, dull, apathetic”).
Choler/Yellow bile
Choler of course brings to mind the disease cholera, an infection of the small intestine. Also a 14th-century word, it originally meant “bilious of temperament or complexion”—with bilious here meaning that you looked a bit green or queasy thanks to the Greek root khole or “bile,” which in turn came from khloros, “pale green, greenish-yellow.”
Choleric then came to mean “easily angered or hot-tempered” by the late 1500s. Thus, people deemed to have too much yellow bile were thought to be irritable and quick to anger. This humor was associated with summer, youth, fire, warmth and dryness, and was thought to originate in the spleen (hence “spleen” as a word for anger).
Melancholy/black bile
You’ll notice that same chol syllable in “choleric” and “melancholy,” and that’s no coincidence. Literally meaning “black bile,” melancholy is comprised of the Greek melan, or “black” (which you might recognize in the word melanin, which causes melanism in animals as well) + khole.
Naturally, people with too much black bile were thought to suffer from sadness and depression. Associated with autumn, adulthood, coldness and dryness, and the element earth, it was thought to originate in the gallbladder. (Does that mean I can cure my depression by getting my gallbladder removed? I kid, I kid.)
After medical science progressed and humorism was proven to be a rather ineffective means of diagnosing… well, most things, the different humors led to the use of the word as a general concept meaning “mood, state of mind” in the 1520s.
It wasn’t until the 1680s that the word humor began to refer to something funny, comedic or amusing. This meaning evolved from the sense of “humor” as a word for “mood” (e.g., “I am in an ill-humor/good humor.”), which then came to refer to both “humoring” someone’s mood or whim, and then came to refer to something that could alter your mood by making you laugh—such as a joke or a comedy play.
This table from the 1926 Modern English Usage by H.W. Fowler offers an interesting way to distinguish different types of humor in the comedic sense (wit, sature, sarcasm, etc.).
Useless Etymology 