Nightmares occur at night, but what does the mare part mean? The word (probably modeled after similar examples in Dutch and German) was first used in English around the start of the fourteenth century CE, when it was hyphenated. Mare in this context was an old word referring to a type of incubus that was thought to possess people while they slept and give them bad dreams (and in case you were wondering, this was completely unrelated to our modern term for "female horse" or the Latin word for "sea"). That comes from the Proto-Germanic word maron and Proto-Indo-European maro, both of which still meant "incubus". Finally, the night- part of nightmare is less interesting: it comes from Old English niht, Proto-Germanic nahts, and Proto-Indo-European newkt, which all had the same definitions.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.