Is it ensure, insure, or assure?
late 13c., "an evil female spirit afflicting sleepers with a feeling of suffocation," compounded from night + mare (3) "goblin that causes nightmares, incubus." Meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" first recorded 1829; that of "very distressing experience" is from 1831. Cognate with Middle Dutch nachtmare, German Nachtmahr.
nightmare night·mare (nīt'mâr')
A dream arousing feelings of intense fear, horror, and distress.
An event or experience that is intensely distressing.