`Nightmare’

Dreams can be pleasant or unpleasant. Nightmares, on the other hand, are always unpleasant. A frightening dream is usually called a nightmare. When you’ve had a nightmare, you wake up gasping for breath. The `mare’ in `nightmare’ has nothing to do with a female horse. The word comes from Old English `mare’ meaning `incubus’. An incubus, people believed, was an evil spirit which sat on a person’s chest while he slept — hence, the feeling of suffocation.

Source: ‘Know Your English’ (The Hindu) – August 07, 2006