Suddenly, he pauses the show and asks me about the word "pesadilla" (nightmare). It sounds weird in Spanish, doesn´t it? Well, it´s a common word, so it doesn´t sound weird to me. What are the origins of the word, do you know? he asks.
We look it up. I found a book that explains this perfectly. Sleep Paralysis: Night-mares, Nocebos, and the Mind-Body Connection By Shelley Adle:
The identifying features of the night-mare are evident in the terms used to refer to it. The etymology of the English word nightmare, for example, reveals a great deal about the experience itself. "Mare" comes from the same root as the German mahr and Old Norse mara, a supernatural being - usually female - who lay on people´s chests at night, suffocating them. The specific terms for night-mare that are used in many contemporary cultures are etymologically related to words for "weight" and "pressing". Mare appears to be of Indo European origins, although its initial meaning is not clear. Linguists purpose three possible roots of the word: noros (death), mer (drive out), and, perhaps the most likely source, mar (to pound, bruise, crush). Because the sense of pressure or weight is prominent in the night-mare experience, it is not surprising that it is also a key element in the historical development of its linguistic forms. The idea of pressure is also present in other terms for the night-mare experience that do not share the mare linguistic root. The medieval French appesart and the Spanish pesadilla, for example, are both derived from the verb peser, meaning "to press down upon".
It turns out people have always thought nightmares were heavy, and their languages have indicated as much.