nightmarish

The word 'nightmarish' is a modern English formation, directly combining the existing noun 'nightmare' with the adjectival suffix '-ish'. The noun 'nightmare' itself has a vivid history, originating from the Middle English nightmare, where 'night' is self-explanatory and 'mare' referred not to a horse but to a malevolent supernatural entity in Germanic folklore believed to torment sleepers. The suffix '-ish', of Old English origin, is a prolific adjective-forming element meaning "of the nature of." Therefore, 'nightmarish' literally means "having the terrifying and oppressive qualities of a visit from the night-mare spirit," a concept which evolved seamlessly to describe anything intensely frightening, distressing, or surreal, much like a bad dream.

Structure: nightmare + ish - nightmare (English compound): A frightening or unpleasant dream. From Middle English nightmare, from night + mare (an evil spirit thought to lie upon and suffocate sleepers). - ish (English suffix): A suffix forming adjectives with the sense "having the qualities of," "somewhat," or "belonging to." From Old English -isc.