How people named longing, sadness, fear and wanting.
nightmare (n.)
Collection
The Inner Life
A nightmare is a bad dream. The mare in it is an old word for a spirit or demon once believed to trouble sleeping people. The horse is a different word.12
In the old belief, the mare oppressed the sleeper and brought a feeling of suffocation. The word later moved from the creature to the experience, and then to a frightening dream.2
The creature disappeared from belief, but the word stayed. Related forms survive in other Germanic languages too.2
It was a nightmare.