• Prone to sudden, unexpected, and acute shifts in mood or demeanor; energetic and frequently changing

  • (sometimes capitalized) Relating to the planet Mercury, or being born under an astrological sign that bears its influence

  • (sometimes capitalized) Relating to the Roman god Mercury

  • Relating to the element mercury or its effects


  • A medicine containing mercury


Have you ever heard the phrase “mad as a hatter” used to describe someone who’s eccentric or prone to drastic mood swings? The phrase is likely a reference to the psychological effects of prolonged mercury exposure experienced by 18th and 19th century hat-makers working with the substance to produce felt (a sturdy fabric). Besides being one of the most whimsical sayings ever inspired by a horrifying health condition, “mad as a hatter” can be perfect for characterizing people who are excessively mercurial. Probably best to say it out of earshot, though — like with mad hatters, you can’t predict how mercurial individuals will react.

Mercurial depicts something or someone as susceptible to sudden and sweeping changes. The word is especially useful for describing people who are liable to frequent and unpredictable mood or behavior swings. For example, even if a mercurial professor is energetic and engaging at the beginning of a lecture, she may abruptly become boring, distracted, or meaner and more likely to assign homework at any point. Accordingly, her students might warn their peers that her lectures and teaching style are mercurial because they shift so quickly. The word can also characterize people who exhibit frequent changes in their abilities or performances; for instance, sports reporters commonly use mercurial when describing particularly inconsistent athletes.

Describing someone as mercurial can also imply that he or she is vivacious or very energetic, which can certainly explain a tendency to change rapidly and unexpectedly. When used in this way, the word essentially indicates that someone shares qualities with Mercury, the Roman god of merchants and thieves (mercurial actually derives from the name Mercury). In Roman mythology, Mercury is clever, eloquent, and lively, and so mercurial people are often understood to be as well. The word is sometimes capitalized when used this way, especially when describing something that’s directly related to Mercury, such as his Mercurial winged sandals or a Mercurial festival. If you’re zodiacally inclined, you can also use this sense of Mercurial (again, often capitalized) to say that something pertains to or is astrologically influenced by the planet Mercury. Those born under Mercury, in other words, those with Mercurial astrological signs, are supposedly more likely to be animated and sprightly. Don’t stress too much over choosing between these two senses, though. Because the planet Mercury is named after the Roman god, its astrological influence purportedly reflects his characteristics. As a result, both of these meanings of mercurial attribute the same qualities of energy and mental alertness.

Finally, mercurial can indicate that something is related to or contains the metallic element mercury. Any of mercury’s distinguishing features, such as its silvery color or that it is liquid at room temperature, would be a mercurial property. Because mercurial ointments (i.e. ointments that contained mercury) were once administered to treat syphilis, you could say that 19th-century syphilis patients often exhibited mercurial symptoms — in other words, they exhibited symptoms of mercury poisoning. Doctors and pharmacists also sometimes use mercurial as a noun to refer to a drug that contains mercury. Modern medicine has seen most mercurials supplanted by safer alternatives, though some vaccines, over-the-counter medications, and dental amalgams still contain mercury compounds.

Example: His mercurial manner made him a dynamic and inspiring leader, but his employees often found his volatility intimidating.

Example: Though he showed flashes of talent, the mercurial striker was never able to sustain his form.

Example: Her Mercurial sign supposedly gave her an energetic and outgoing streak, but she was really quite reserved.

Example: Because of Mercury’s dominion over commerce, some Roman merchants practiced mercurial rites to ensure good business.

Example: Broken mercurial thermometers pose serious health threats.

Example: Cinnabar is a mercurial commonly used in traditional Chinese medicine.


When mercurial first entered English in the late 1300s, it was used to signify relation to the planet Mercury. By the late 1500s, the word’s meaning had broadened to include its current senses of “lively” and “prone to rapid change,” both of which originally reflected the supposed astrological influence of the planet Mercury. Mercurial in English was borrowed from the Old French mercurial, which in turn was borrowed from the Latin mercurialis. The latter term comes from Mercurius, the Latin word for the god Mercury, after whom the planet Mercury is named and who embodied vivacity, eloquence, and changeability. The name of the deity, who was said to preside over trade, comes from the word merx, meaning “goods” or “merchandise.”

Derivative Words

Mercurially: This adverb of mercurial notes when an action is done in a sprightly or moody fashion or when an adjective is related to unpredictable changeability.

Example: The region’s weather changes are known for being mercurially rapid.

Example: She leaped mercurially onto the train, eager to begin her journey.

Mercuriality: Mercuriality indicates a lively or quickly changing nature.

Example: His mercuriality made him a lively conversation partner, but his friends found him difficult to deal with at times.

Mercurialness: This additional noun form of mercurial serves the same function as mercuriality, describing a state of animation or moodiness.

Example: Her mercurialness made her an engaging actress, as she could nimbly navigate the emotional landscape of her characters.

In Literature

From David Lovelace's Scattershot: My Bipolar Family:

Depression is a painfully slow, crashing death. Mania is the other extreme, a wild roller coaster run off its tracks, an eight ball of coke cut with speed. It’s fun and it’s frightening as hell. Some patients - bipolar type I - experience both extremes; other - bipolar type II - suffer depression almost exclusively. But the “mixed state,” the mercurial churning of both high and low, is the most dangerous, the most deadly. Suicide too often results from the impulsive nature and physical speed of psychotic mania coupled with depression’s paranoid self-loathing.

In this instructive passage on the hazards of living with bipolar disorder, Lovelace describes the terrifying oscillation between mania and depression that some bipolar disorder sufferers face. What make it so dangerous, he relates, are the unpredictable and drastic, or mercurial, swings between the two extremes that may result in a state of mind susceptible to suicide.


  • It’s hard to cure a mercurial person of their volatility!

  • A mercurial demeanor can’t curtail rising emotions.

  • A mercurial person’s mood moves around like a ball of mercury.


Myth, Religion, Astrology, Mood, Character, Emotion, Behavior, Mercury, Chemistry

Bring out the linguist in you! What is your own interpretation of mercurial. Did you use mercurial in a game? Provide an example sentence or a literary quote.