IPA: /ˈfætɪli/
KK: /ˈfætɪli/
In a manner that is characteristic of or resembles fat; in a fatty way.
The food was cooked fattily, making it rich and heavy.
Fattily → It is formed from "fat" (from Old English "fǣtt", meaning having a large amount of body fat) and "-ily" (a suffix used to form adverbs meaning in a manner of). The word "fattily" means in a manner that is fat or characterized by fatness.
Think of the word 'fat' describing something with a lot of body fat, and the '-ily' shows how it is done — so 'fattily' means doing something in a fat manner.