A fiend or incubus formerly supposed to cause trouble in
sleep.

A condition in sleep usually caused by improper eating
or by digestive or nervous troubles, and characterized by a sense of
extreme uneasiness or discomfort (as of weight on the chest or stomach,
impossibility of motion or speech, etc.), or by frightful or oppressive
dreams, from which one wakes after extreme anxiety, in a troubled state
of mind; incubus.

Hence, any overwhelming, oppressive, or stupefying
influence.