IPA: /ˈstjuːpɪfaɪɪŋli/
KK: /ˈstjuːpɪfaɪɪŋli/
In a way that causes someone to be unable to think clearly or to be amazed.
The magician performed stupefyingly complex tricks that left the audience in awe.
Stupefyingly → It is formed from "stupefacere" (from Latin, meaning to stun or make numb) and the suffix "-ly" (meaning in a manner). The word "stupefyingly" means in a manner that stuns or astonishes.
Think of being 'stunned' ('stupefacere') and how something can happen 'in a manner' ('-ly') that leaves you astonished.
No commonly confused words.