IPA: /ˈstæɡərɪŋli/
KK: /ˈstæɡərɪŋli/
In a way that is surprising or shocking, often because it is very large or extreme.
The movie was staggeringly good, leaving the audience in awe.
Staggeringly → It is formed from "stagger" (from Old English *stacgan*, meaning to sway or totter) and the suffix "-ingly" (meaning in a manner of). The word "staggeringly" describes something done in a manner that causes one to sway or be astonished.
Think of something that makes you 'sway' or feel 'astonished' — that's why 'staggeringly' means done in a way that causes great surprise.