In a way that causes fear, dread, or a sense of cold; in a manner that is unpleasantly cold or gives someone chills.
Chilling (present participle of chill) + -ly (adverb-forming suffix); creates an adverb describing how something is done in a frightening or cold way.
Adverbs ending in -ly that come from verbs are fascinating because they let you describe how something happens—'chillingly' captures both the literal shivers and the emotional fear perfectly.
Complete word intelligence in one call. Free tier — 50 lookups/day.