IPA: /ˈtɛrɪfaɪɪŋli/
KK: /tɛrɪfaɪɪŋli/
In a way that causes great fear or dread.
The movie was terrifyingly suspenseful, keeping everyone on the edge of their seats.
Terrifyingly → It is formed from "terrify" (from Latin "terrere", meaning to frighten) and the adverbial suffix "-ingly" (meaning in a manner of). The word "terrifyingly" means in a manner that causes fright or terror.
Think of the root 'terrify' which means to frighten, and remember that 'terrifyingly' describes something done in a frightening way.