IPA: /naɪtˈmɛrɪʃli/
KK: /naɪtˈmɛrɪʃli/
In a way that is very frightening or unpleasant, like a nightmare.
The movie was nightmarishly scary, leaving the audience in shock.
Nightmarishly → It is formed from "nightmare" (from Old English *nīhtmære*, meaning a bad dream) and the suffix "-ishly" (meaning in a manner of). The word "nightmarishly" describes something that is done in a manner reminiscent of a nightmare, often implying something frightening or distressing.
Think of a 'nightmare' as a bad dream, and remember that 'nightmarishly' means something that feels like it comes from a bad dream.
No commonly confused words.