Nightmare is a half understood word in the sense that the first half is clear, but the second half is not. What is a mare in this context and what does it have to do with waking up in the middle of the night with your heart pounding? As it turns out, both halves of the word are interesting and both have remarkably deep and consistent Indo-European roots.
Night is derived from the Proto-Indo European (PIE) word *nekwt, meaning “night”. This word is remarkably stable across languages and follows a patter of n + vowel + palatal or velar consonant, generally fricative:
- nacht (German)
- nox (Latin)
- naktam (Sanskrit)
- natt (Swedish)
- naktis (Lithuanian)
It is also seems to me to be connected to two other words, next and near. As is often the case, this connection is a bit clearer in German where the words are nächste and nahe, respectively. Other German words that fit this pattern are nach, which means “to” or “towards” and nachbar, which means “neighbor” in the literal sense of a “near dweller”.
The common thread linking all of these words that follow the form n + vowel + palatal or velar consonant seems to be the idea of proximity or closeness, which fits nicely with the constrictive nature of night and darkness in general.
Now for the second part of the word, mare. This comes from the Old English word mare, meaning an evil spirit or an incubus. This word also has deep and consistent Indo-European roots, with the word being mahr in German and mara, marra or mare in the Scandinavian languages. While there are a couple of competing theories regarding which PIE root mare comes from, the leading contender is *mer, which means “to harm” or “to rub” with the connotation of “chafing”, “rubbing away” or “wearing out”.
Putting the word back together we get to something that seems to indicate an evil spirit in close proximity to the sufferer and putting them under duress with constant, wearying pressure. Not a bad way to describe something that haunts your sleep and terrifies you while you are unconscious.
In the end, I’m left with thought that the original idea as to the cause of nightmares, a literal demon perched atop you while you sleep, pressing on you, wearing you down in the darkness, is nothing more than a poetic way to describe a phenomenon that would today be described in more prosaic terms using scientific psychological language related to things like Post Traumatic Stress Disorder that I don’t think most people understand any better than they understand the term “demon”.
It seems to me that people in both eras attempted to define and describe something complex and poorly understood with terms that were appropriate to their times and cultures. Ultimately, though, they are both using metaphors and, as usual, the older ones seem more vibrant and compelling.