Litmus, as in litmus test, is just one of those words that looks like it’s from Latin. For one, it ends in -us, a signature case ending in the language. For another, many of us first encounter the word in chemistry class, and science, we know, brims with Latin derivatives. So, why don’t we put the word litmus to the etymological litmus test?
Litmus, a blue dye obtained from lichens, has been used by scientists as far back as the Middle Ages. In modern chemistry, it tests for acidity, turning red in the presence of acids and back to blue in alkalis.
Litmus paper appears in the 1803 writings of Cornish scientist Humphry Davy, who gave us sodium and the first modern chemical use of element. The metaphorical litmus test, which indicates something decisively, emerges in the 1950s.
According to the Barnhart Dictionary of Etymology, there are two forms influencing litmus. The first, attested in 1320, is lykemose, borrowed from the Dutch lijkmoes. The first element, lijk-, may either be related to the English lac, a “dark red dye,” which shows up in the Sanskrit-rooted lacquer and shellac, or the common word leak. The second element, –moes, is simply the Dutch cousin of our word moss.
But around the same time, Middle English borrows another word for litmus: litemose, coming in the 1320s from a Scandinavian source related to the Old Norwegian litmosi or Old Swedish letmossa. The latter half, –mose is once again just the Scandinavian sibling of the English moss, while lite- is kin to an now-obsolete, Norse-derived English verb, lit, “to dye” or “stain.”
The Dutch lykemose appears earlier, plus litmus mixtures have long been obtained from lichens grown in the Netherlands. But it seems the Scandinavian litmus spelling prevailed due to the influence of the existing word lit.
Either way, litmus literally means something like “leak-moss,” “lac-moss,” or “dye-moss” – and it definitely doesn’t pass the Latin litmus test.