IPA: /hɪˈdiəsli/
KK: /ˈhɪdiəsli/
In a very ugly or unpleasant way; extremely.
The monster looked hideously deformed, frightening everyone who saw it.
Hideously → It is formed from "hideous" (from Old French *hideus*, meaning frightful or horrible) and the suffix "-ly" (meaning in a manner of). The word "hideously" means in a manner that is frightful or horrible.
Think of something that is 'frightful' ('hideous') and how it is done 'in a manner' ('-ly'). This helps you remember that 'hideously' describes something done in a frightful way.