Resembling or having the quality of a nightmare; extremely frightening, disturbing, or unpleasant.
From 'nightmare,' combining 'night' and an old word 'mare' (a female demon or spirit) believed to cause bad dreams. The suffix '-ish' was added to mean 'resembling' or 'having qualities of.' The demon sense dates back to medieval folklore.
People used to genuinely believe a 'mare' (evil spirit) sat on their chest while sleeping and caused nightmares—the word 'nightmare' literally means 'night-demon'—and this same creature appears in folklore across many European cultures under different names!
Complete word intelligence in one call. Free tier — 50 lookups/day.