IPA: /həˈrɪfəˌɪŋli/
KK: /hɔːrɪfaɪɪŋli/
In a way that causes great fear or shock.
The movie was horrifyingly realistic, making everyone in the audience scream.
Horrifyingly → It is formed from "horrific" (from Latin "horrificus", meaning causing horror) and the adverbial suffix "-ly" (meaning in a manner of). The word "horrifyingly" means in a manner that causes horror or dread.
Think of something that causes 'horror' ('horrific') and remember that 'horrifyingly' describes how something can be done in a way that evokes that fear.