It’s no coincidence that water and wet not only have closely related meanings but also have similar spellings. Both words spring from the same prehistoric Indo-European root, *wed-, whose basic meaning was “water.”
Historical linguists have managed to reconstruct many Indo-European word stems based on *wed-. Part of the distinction between these forms has to do with their vowels; Indo-European often used such a change in vowels, or ablaut, between different forms of a word, just as English uses different vowels to express tense in irregular verbs such as write/wrote/written or give/gave/given. Additionally, Indo-European roots could take various suffixes to express shades of meaning.Thus water comes from the stem *wod-ōr, in which the vowel of wed- has shifted to o and a suffix has been added, while wet comes from the stem *wēd-o-, with a lengthened vowel and a different suffix. Still another vowel change (plus a suffix) turned *wed- into *ud-ōr . In the branch of Indo-European that eventually became Greek, this last stem remained nearly unchanged for thousands of years—hudōr means “water” in classical Greek. In turn hudōr is the source of nearly all the hydr- words in English having to do with water: hydrant, hydration, hydraulics, hydroelectric, hydrogen, and so on.
Nearly all. But not quite all. The hydra, a many-tentacled microscopic animal living in freshwater habitats, is named for the Hydra, a many-headed aquatic monster in Greek mythology. The Hydra’s name is indeed from the *wed- root, but it comes not from the stem *ud-ōr but from *ud-ro- or *ud-rā- , which had the basic meaning “water animal.” In the Germanic branch of Indo-European this stem eventually evolved into the English otter and various closely related cognate forms (“otter” is Otter in German, oddere in Danish, uttrar in Swedish), while in Latin the same stem acquired an l and became lutra—to this day Lutra lutra is the scientific name for the European otter. Apparently in some colloquial dialects of Latin lutra became modified even further, with the l becoming an n. By the time the dialect spoken in the Roman provinces of Hispania evolved into what we now know as Spanish, the word for “otter” in that language had become nutria. A few centuries later, Spanish explorers in South America applied the same word to some very large aquatic rodents they found there, even though those rather tubby, scrawny-tailed creatures were not much like otters at all aside from living in rivers and having furry coats. The rodents themselves have since become naturalized in the United States, and their name has likewise become naturalized into English; if you take a canoe out for a paddle in the marshes of southern Louisiana, you’re at least as likely to see a nutria as an otter. (You won’t see a hydra at all, unless you bring a microscope!)
*Here and in the dictionary, we use an asterisk to indicate that the root, stem, or word in question is not attested in any surviving documents but has been hypothetically reconstructed by linguists based on known forms and grammatical principles.
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