IPA: /ˈsuːtɪli/
KK: /ˈsutɪli/
In a manner that is covered with or resembles soot, which is a black powdery substance produced by burning organic matter.
The chimney swept the sootily blackened walls clean after the fire.
Sootily → It is formed from "soot" (from Old English *sōt*, meaning a black powdery substance produced by burning organic matter) and "-ily" (a suffix used to form adverbs meaning in a manner of). The word "sootily" means in a manner that is covered with or resembles soot.
Think of the black powdery substance ('soot') and how something can be done 'in a manner of' ('-ily') being covered in it. This helps you remember that 'sootily' describes something done in a way that involves soot.