In my favorite coffee shop this week, the owner had a box on the counter with the label “please return thermoses here.” We started speculating on the correct plural of thermos, but we didn’t come up with an answer.

Thermos (still a trademark name in Britain) was registered in 1907. The thing itself was invented by the Scottish scientist Sir James Dewar, who, among other accomplishments, first liquified hydrogen, was co-inventer of cordite, made a bubble four feet across, and was the first to predict what is now called superconductivity.

His discovery of the vacuum flask was a side-effect of his research in cryogenics. He built the first one in 1892, but no one bothered to manufacture them commercially until 1904, when two German glass blowers formed Thermos GmbH. The thermos itself was patented then, but not so named until three years later. Supposedly the company sponsored a contest to name the thing, and a Munich resident won with a submission of Thermos.

The trouble with pluralizing it is that thermos is an ancient Greek adjective (it literally means “hot”). English thermos at first generally was used as an adjective, too. People wrote of a “Thermos flask” or “Thermos bottle.” That makes the formation of a proper plural somewhat uncertain, as the ending in Greek might be different depending on whether you treat thermos as a noun or an adjective, and depending on the gender of the noun it is attached to.

Thermoses is how it usually has appeared in English, but the coffee shop owner and I agreed that doesn’t look or sound right. Thermae or therma are more etymologically correct, but they look nothing like English.

Greek thermos is related to therme “heat,” from the Proto-Indo-European base *ghwerm-/*ghworm- meaning “warm.” It’s relatives include Latin fornax “an oven, kiln;” Sanskrit gharmah “heat;” and Old English wearm (modern warm).

In ancient Persia, today would have fallen in the month of Garmapada a name formed from garma- “heat,” which is yet another thermos cousin.

Greek thermos meant both “warm” and “hot.” The distinction, based on degree of heat, between “warm” and “hot” is general in Balto-Slavic and Germanic languages, but in other tongues one word often covers both concepts, for instance Latin calidus, French chaud, Spanish caliente.

Greek nouns commonly end in -s in the singular, but -s is the overwhelming choice for pluralizing nouns in English. This has confused English-speakers who encounter words nativized from Greek. One common reaction is to snip off the -s and make a false English singular out of a perfectly good Greek one. The gyro sandwich that you get at a Greek restaurant, for instance, is really a gyros in Modern Greek (the word means “circle,” and the name originally referred to the roasted lamb in the sandwich, which was cooked on a rotating spit).

Kudos meaning “credit or praise” is another Anglicized Greek singular noun in -s, and the false singular kudo sometimes turns up in English. But kudos is rare in English, and its use generally is restricted to learned circles. Thus pedants can keep it on a tight leash and pounce on anyone boorish enough to write kudo.

The more common problem is the one presented by thermos: how to pluralize a noun that already ends in -s. Rhinoceros (from a Greek compound meaning “nose-horn”) has puzzled people for generations. “What is the plural of rhinoceros?” Sir Charles Eliot wrote in “The East Africa Protectorate” in 1905. “Well, Liddell and Scott [who wrote the authoritative Greek-English Lexicon] seem to authorize ‘rhinocerotes,’ which is pedantic, but ‘rhinoceroses’ is not euphonious.”

Fortunately, in that case, English as embraced the easily pluralized short form rhino. No such solution is offered for octopus, however, which is from the Greek adjective oktopous, meaning “eight-footed.” The proper plural would be octopodes, though octopuses probably works better in English. Octopi is an ignorant error, from mistaken assumption that the -us in the word is the Latin noun ending that takes -i in plural.

Another puzzler is biceps, which is a Latin word meaning “having two parts” (literally “two-headed”). Despite the -s, it is singular, and there is no such word as *bicep. A proper plural would be bicepses.

Perhaps the most astonishing relative of thermos is fornication, which comes ultimately from Latin fornax “oven.” Because ancient ovens were arched or dome-shaped, the word (in slightly altered form fornix) came to mean “arch, vaulted chamber.” And because Roman prostitutes commonly solicited from under the arches of certain buildings, the word acquired yet another layer of meaning, “brothel.” From there it was just a short hop to fornication.

I often waste bandwidth this way over at the other site. I don’t know if it’s of any interest here.