IPA: /əˈpɔːlɪŋli/
KK: /əˈpɔlɪŋli/
In a way that causes shock or dismay; extremely bad or unpleasant.
The service at the restaurant was appallingly slow, making us wait for over an hour.
Appallingly → It is formed from "appall" (from Old French *apalir*, meaning to make pale or to frighten) and the suffix "-ingly" (meaning in a manner). The word "appallingly" describes something done in a manner that causes shock or dismay.
Think of something that makes you feel 'frightened' or 'pale' ('appall') and remember that 'appallingly' means it is done in a shocking way.