Build your vocabulary with Merriam-Webster's Word of the Day! Each day a Merriam-Webster editor offers insight into a fascinating new word -- explaining its meaning, current use, and little-known details about its origin.
Merriam-Webster's Word of the Day for September 23, 2023 is: tenebrous \TEN-uh-brus\ adjective
Tenebrous is a formal word that is often used as a synonym of [gloomy](https://www.merriam-webster.com/dictionary/gloomy). It also can be used to describe dark, unlit places (as in “the tenebrous abyss”) or things that are difficult to understand (as in “a tenebrous tangle of lies”).
// The neighborhood children made sure never to approach the abandoned mansion, which sat tenebrous and foreboding at the top of the hill.
// A horror film seems incomplete without someone running through a tenebrous forest or alley.
[See the entry >](https://www.merriam-webster.com/dictionary/tenebrous)
“On the heels of Greig Fraser’s spectacular work on Dune, the cinematographer gives the film a moody, tenebrous look to match the tortured pit of Batman’s soul, and production designer James Chinlund’s world-building is first-rate, weaving together elements from real cities and sets to form a Gotham that resembles New York while establishing its own gritty, gothic identity, pulsing with menace and mystery.” — David Rooney, The Hollywood Reporter, 28 Feb. 2022
Did you know?
Tenebrous can mean both “obscure” and “murky,” but its history is crystal clear. Etymologists know that the word comes from the Latin noun tenebrae, meaning “darkness.” Tenebrous has been used in English since the 15th century, and in subsequent centuries has been joined by some interesting and even less common relations. [Tenebrionid](https://www.merriam-webster.com/dictionary/tenebrionid) is the name that may be given to any of at least 20,000 species of mostly nocturnal beetles, also called darkling beetles, many of whom love inhabiting dark places. [Tenebrism](https://www.britannica.com/art/tenebrism) refers to a style of painting—associated especially with the Italian painter [Caravaggio](https://www.merriam-webster.com/dictionary/Caravaggio)—in which most of the figures are engulfed in shadow while some are dramatically illuminated by concentrated light. And let’s not forget the terrific [tenebrific](https://www.merriam-webster.com/dictionary/tenebrific), a tenebrous synonym.
Merriam-Webster's Word of the Day for September 22, 2023 is: mesmerize \MEZ-muh-ryze\ verb
Mesmerize means "to hold the attention of someone entirely; to interest or amaze someone so much that nothing else is seen or noticed." The word is often used in the phrase "be mesmerized."
// The crowd was mesmerized by the flawlessly synchronous movements of the acrobats.
[See the entry >](https://www.merriam-webster.com/dictionary/mesmerize)
"Yep, Ruth [Handler] ended up naming two of her iconic dolls after her kids. The idea for Barbie and Ken stemmed from a family Europe trip in 1956.... Barbara, then still a teenager, saw a doll that looked like an adult woman in a store window in Switzerland and was mesmerized." — Korin Miller, Women's Health, 21 July 2023
Did you know?
Experts can’t agree on whether [Franz Anton Mesmer](https://www.britannica.com/biography/Franz-Anton-Mesmer) (1734-1815) was a quack or a genius, but all concede that the [Swabian](https://www.merriam-webster.com/dictionary/swabian) physician's name is the source of the word mesmerize. In his day, Mesmer was the toast of Paris, where he enjoyed the support of notables including Queen Marie Antoinette. He treated patients with therapeutic procedures (called, appropriately enough, [mesmerism](https://www.merriam-webster.com/dictionary/mesmerism)) involving what he claimed was a mysterious force termed animal magnetism. (Many believe that mesmerism was what we now call [hypnotism](https://www.merriam-webster.com/dictionary/hypnotism)). Accordingly, the verb mesmerize was first used to mean "to subject to mesmerism" before broadening to be synonymous with [hypnotize](https://www.merriam-webster.com/dictionary/hypnotize), and later to mean "to amaze or captivate."
Merriam-Webster's Word of the Day for September 21, 2023 is: regimen \REJ-uh-mun\ noun
Regimen refers to a plan or set of rules about food, exercise, etc., designed to make someone become or stay healthy.
// Sherry’s personal trainer at the gym started her on a workout regimen of 30 minutes on the treadmill followed by 30 minutes of weight training.
[See the entry >](https://www.merriam-webster.com/dictionary/regimen)
“For those with natural hair, taking on a protective hairstyle is more than an expectation, it’s a symbolic rite of passage. ... That said, tucking your hair into a protective style is not an excuse to completely disregard all hair-care practices. If anything, it's the exact opposite: Establishing an effective hair-care regimen is essential to maximizing and maintaining a protective style, so once it’s removed, both the scalp and hair are healthy and happy.” — Janelle Sessoms, Fashionista.com, 16 June 2023
Did you know?
Being but humble lexicographers, we cannot say whether an apple a day truly keeps the doctor away, but as far as regimens go, one could do a lot worse than snackin’ on a McIntosh. Regimen, which usually refers to a system of rules or guidelines—often for living a healthy life or taking a regular dose of exercise—comes ultimately from a Latin verb, regere, meaning “to direct.” Regere led in apple-pie order to the English word regimen, first by way of the Latin noun regimen, meaning “steering” or “control,” and then via the Medieval Latin regimen, referring to a set of rules. Other regere descendants fell further from the tree, including [correct](https://www.merriam-webster.com/dictionary/correct), [erect](https://www.merriam-webster.com/dictionary/erect), [region](https://www.merriam-webster.com/dictionary/region), [rule](https://www.merriam-webster.com/dictionary/rule), and [surge](https://www.merriam-webster.com/dictionary/surge). Be sure not to confuse regimen with another of its kin, [regiment](https://www.merriam-webster.com/dictionary/regiment), which refers to a military unit, as doing so could upset the apple cart.
Merriam-Webster's Word of the Day for September 20, 2023 is: churlish \CHUR-lish\ adjective
Churlish is a formal word that means “irritable and rude.”
// It would be churlish not to congratulate the winning team because we lost the match.
[See the entry >](https://www.merriam-webster.com/dictionary/churlish)
“‘Ted Lasso’ has gradually become more of a light drama than a comedy, but it’s such a pleasant one that it seems churlish to even point this out. In that dramatic vein, the show's depiction of Nate is more compelling than I might have anticipated. The series has never been particularly interested in validating the man-child archetype, but it is interested in how insecurity can manifest itself into toxic behavior and Nate is the epitome of that.” — Nina Metz, The Chicago Tribune, 15 Mar. 2023
Did you know?
In Old English, the word [ceorl](https://www.merriam-webster.com/dictionary/ceorl) referred to a free peasant—someone who was neither part of the nobility nor enslaved or in debt. In Anglo-Saxon England, which lasted roughly from the 5th to 11th centuries, [ceorls](https://britannica.com/topic/ceorl) had many rights that peasants of lower social status did not, and a few even rose to the rank of [thane](https://www.britannica.com/money/topic/thane-feudal-lord). However, as most ceorls were driven into the class of unfree [villeins](/dictionary/villein) over the centuries, especially following the [Norman Conquest](https://www.britannica.com/event/Norman-Conquest), the connotation of the word ceorl—spelled cherl in Middle English and then finally churl—diminished as well, eventually coming to mean “a lowly peasant” and later “a rude, ill-bred person.” Similarly, churlish began in the form ceorlisc in Old English as a simple descriptor of someone with the rank of ceorl, but today it describes a [boorish](https://www.merriam-webster.com/dictionary/boorish) person, or their rude and insensitive behavior.
Merriam-Webster's Word of the Day for September 19, 2023 is: pontificate \pahn-TIF-uh-kayt\ verb
To pontificate is to speak or express an opinion about something in a [pompous](https://www.merriam-webster.com/dictionary/pompous) or [dogmatic](https://www.merriam-webster.com/dictionary/dogmatic) way.
// Stan loves to hear himself talk and will often pontificate on even the most trivial issues.
[See the entry >](https://www.merriam-webster.com/dictionary/pontificate#h1)
"Fact is, you can find good pizza from Memphis to Salt Lake City. But you have to look a lot harder than you do in Orlando. So, stop with this nonsense already. Similarly, let's abandon the absolutes. This place is THE BEST. That place is THE WORST. These things are entirely subjective and ranted about on the internet by a small but exhaustingly vocal contingent of zealots, many of whom I suspect enjoy pontificating far more than they enjoy pizza." — Amy Drew Thompson, The Orlando (Florida) Sentinel, 8 June 2023
Did you know?
We hate to drone on, so we’ll give you the [TL;DR](https://www.merriam-webster.com/dictionary/tldr) on pontificate. In ancient Rome, a [pontifex](https://www.britannica.com/topic/pontifex) (plural pontifices) was a member of an important council of priests. With the rise of Catholicism, the title pontifex was transferred to the Pope and to Catholic bishops. From pontifex, by way of Medieval Latin, comes the English verb pontificate, which in the early 1800s meant “to officiate as a [pontiff](https://www.merriam-webster.com/dictionary/pontiff)”—that is, as a bishop or Pope. (Note that the noun pontificate), which refers to the state, office, or term of office of a pontiff had been borrowed directly from Latin in the 15th century.) By the late 1800s, pontificate was also being used derisively for lay individuals who spoke as if they had the authority of a member of the clergy. To this day the word connotes an air of [spurious](https://www.merriam-webster.com/dictionary/spurious) superiority—one might consider this sense of pontificate to be the spiritual forerunner of [mansplain](https://www.merriam-webster.com/dictionary/mansplain).
Merriam-Webster's Word of the Day for September 18, 2023 is: zenith \ZEE-nith\ noun
Zenith refers to the strongest or most successful period of time for a person or thing.
// At the zenith of her music career in the early 2000s, she released her best-selling album to date.
[See the entry >](https://www.merriam-webster.com/dictionary/zenith)
"Once deemed ‘one of the most underrated musicians in rock history’ by David Bowie, John Cale is best known as the viola-scraping Velvet Underground co-founder who grounded the group in the avant-garde. But those years hardly marked a creative zenith for Cale. Since leaving the band in 1968, he has released more than a dozen solo albums, ranging in style from orchestral pop to new wave and punk; collaborated with luminaries like Patti Smith and Brian Eno; and scored numerous films." — Olivia Horn, The New York Times, 18 Aug. 2023
Did you know?
When you reach the zenith, you're at the top, the pinnacle, the summit, the peak. Zenith developed from an Arabic phrase meaning "the way over one's head," and then traveled through Old Spanish, Medieval Latin, and Middle French before arriving in English. As long ago as the 1300s, English speakers used zenith to name the highest point in the celestial heavens, directly overhead. By the 1600s, zenith was being used for other high points as well. The celestial term is often contrasted with nadir, which refers to the point that is vertically downward from the observer (imagine a line going through the Earth from the observer's feet and out the other side into the sky). Figuratively, [nadir](https://www.merriam-webster.com/dictionary/nadir) simply means "the lowest point."