1. in a way that is extremely frightening, disturbing, or unpleasant like a nightmare

Examples

• The abandoned hospital was nightmarishly dark and silent.
• Traffic moved nightmarishly slowly during the storm.
• The test was nightmarishly difficult for most students.
• The old house creaked nightmarishly in the wind.
• The situation became nightmarishly complicated.
• The movie was nightmarishly realistic in its depiction of war.