IPA: /ˈmɒnstrəsli/
KK: /ˈmɑnstrəsli/
In a way that is extremely large, frightening, or evil.
The creature roared monstrously, sending shivers down our spines.
Monstrously → It is formed from "monstrum" (from Latin, meaning a warning or an omen, often associated with something unnatural or abnormal) and the suffix "-ously" (meaning in a manner of). The word "monstrously" means in a manner that is unnatural or abnormal, often implying something frightful or extreme.
Think of something that is an 'omen' or 'warning' ('monstrum') and how it can be perceived as frightening or extreme when described as 'monstrously'.