IPA: /ˈskeɪðɪŋli/
KK: /ˈskeɪðɪŋli/
In a way that is very critical or harsh, often expressing strong disapproval.
The critic spoke scathingly about the movie, pointing out its many flaws.
Scathingly is derived from "scathe" (from Old English "sceað", meaning harm or injury) and the suffix "-ingly" (indicating a manner of doing something). The word describes doing something in a manner that causes harm or criticism.
Think of 'scathe' meaning harm, and remember that 'scathingly' describes doing something in a harmful or critical way.