IPA: /ˈfraɪtənɪŋli/
KK: /ˈfraɪtənɪŋli/
In a way that causes fear or alarm.
The movie was frighteningly realistic, making everyone jump at the scary scenes.
Frighteningly → It is formed from "frighten" (from Old English "fyrhtan", meaning to cause fear) and "-ingly" (a suffix used to form adverbs indicating manner). The word "frighteningly" means in a manner that causes fear or alarm.
Think of causing 'fear' ('frighten') and doing it in a certain way ('-ingly'). This helps you remember that 'frighteningly' describes how something can cause fear.