means Frightening, distressing dreams that jolt you awake or linger uneasily after sleep — and, by extension, any waking situation that feels just as horrible.

from From Old English 'mare' (or 'maere'), a name for an evil spirit or goblin once believed to sit on a sleeper's chest and press the breath out of them — leaving you suffocated and terrified in your sleep. The 'night' was simply tacked on for time of attack. The 'mare' here has nothing to do with horses; it's a separate word, possibly related to Germanic and Slavic terms for crushing demons (the same root behind the French 'cauchemar'). Only later did 'nightmare' drift from 'chest-sitting demon' to 'bad dream' to 'anything dreadful.'