nightmare: [13] The mare of nightmare is not the same word as mare ‘female horse’. It comes from Old English mære, which denoted a sort of evil spirit or goblin which sat on sleepers’ chests and gave them bad dreams. That is what the compound nightmare meant too when it emerged in the early Middle English period, and the metaphorical application to the bad dream supposedly caused by this incubus is not recorded until the mid-16th century. => yell
