IPA: /ˈsuːðɪŋli/
KK: /ˈsuːðɪŋli/
In a way that makes someone feel calm and relaxed.
She spoke soothingly to the frightened child.
Soothingly → It is formed from "soothe" (from Old English *sōthian*, meaning to calm or comfort) and "-ly" (meaning in a manner of). The word "soothingly" means in a manner that calms or comforts.
Think of the word 'soothe' which means to calm, and remember that 'soothingly' describes doing something in a calming way.