IPA: /ˈstɑːrtlɪŋli/
KK: /ˈstɑrtlɪŋli/
In a way that causes surprise or shock; unexpectedly.
The magician performed startlingly amazing tricks that left the audience in awe.
Startlingly → It is formed from "startle" (from Middle English *styrten*, meaning to leap up or jump) and the adverbial suffix "-ly" (meaning in a manner of). The word "startlingly" means in a manner that causes surprise or shock, as if leaping up suddenly.
Think of something that makes you 'jump' or 'leap up' unexpectedly — that's why 'startlingly' describes something done in a surprising manner.