I wondered while walking, a few weeks ago, about the origins of our words dream and nightmare.

It appears that dream is one of those elemental words, that means itself. You know: the succession of images that pass through your mind as you sleep. Or something like that. Dream means dream, whether spoken today, in Middle English dreem, as Old English drēam, Old Norse draumr, even Old High German troum.

You’d think the word for a bad one would be as elemental, and actually it is. What we call a nightmare was originally just a mare (12^th century), or in the several Old tongues: a mera, mara, mære. All of those mean incubus. The Proto Indo European was mora, probably from a base “mer,” meaning to rub away, harm, seize.

You can certainly have a daydream while awake, but unmodified the word suggests sleep in the dark. I wonder why we chose, as our language evolved, to say nightmare instead of just mare, yet never to say nightdream.