IPA: /ˈθrɛtənɪŋli/
KK: /ˈθrɛtənɪŋli/
In a way that causes fear or suggests harm may happen.
The storm clouds gathered threateningly overhead, warning us of the impending rain.
Threateningly → It is formed from "threaten" (from Old English *þreatian*, meaning to threaten or menace) and the suffix "-ingly" (meaning in a manner of). The word "threateningly" describes an action done in a menacing or intimidating manner.
Think of someone acting in a way that shows they are 'menacing' or 'intimidating' — that's what 'threateningly' means.